Sephiroth was always kind: a closer look at the Nibelheim flashback
Back when I first played Final Fantasy 7 sometime in the mid to late 2000s, something I noticed in the Nibelheim flashback was that Sephiroth was actually pretty kind and thoughtful. However, the kind of guys who'd go on to become gamergaters in a few years were very active on the Internet, and if you thought Sephiroth was anything more than a cold, heartless monster, they'd derogatorily call you a yaoi fangirl. Unfortunately, I internalized what they'd said, and for years I distrusted my own senses.
But given that subsequent games in the FF7 franchise have made it abundantly clear that Sephiroth had been a decent person before his breakdown, I'm going to go back and look at the evidence that my younger self spotted, and the evidence that my older self can see more clearly.
First, Cloud says that "After working with Sephiroth on several missions, we became friends." He describes himself and Sephiroth as "war buddies" and says that they "trusted each other." But he also notes that Sephiroth "hardly ever talked about himself."
From this, we know that Sephiroth didn't talk about his problems to either Zack or Cloud. This is appropriate behavior; both of them were subordinate to him, and Cloud was a teenage boy. I think it's also a fair guess that Sephiroth never gave Cloud a reason to feel unsafe or unwanted, because Cloud clearly internalized Zack's stories of being friends with Sephiroth without too much trouble.
It's also been noted that Sephiroth didn't have to ride in the back of the truck with Zack and Cloud. If he was really so detached from everyone that he never cared about anyone else at all, he probably could have gotten a comfier ride. And that's what I'd have Sephiroth do if I was a writer and wanted to establish that he was detached thought himself above everyone else. But instead, we see Sephiroth riding with Shinra's Silliest SOLDIER and Shinra's Snarliest Infantryman.
In the truck, Zack bounces around excitedly. Sephiroth tells him to "settle down," and says that his behavior is "just like a kid." Because the game is pure written dialog with no tags, it's difficult to be 100% sure of Sephiroth's intended tone. Sephiroth might be trying to gently tell Zack that his behavior is inappropriate, or he might find it endearing.
If Sephiroth finds Zack's behavior endearing, that's settles it on Sephiroth caring about other people. If he meant it as criticism, then it's very soft-spoken criticism.
Then, Sephiroth tells Zack "This isn't a typical mission" and Zack responds with "Good!" Sephiroth asks him, "Why do you say that?" He's showing curiosity about Zack's opinions and feelings. If I was a writer and wanted to show that Sephiroth didn't care about anyone, I'd have put far more dismissive and belittling words in his mouth.
Sephiroth doesn't really show a lot of emotion throughout the rest of the conversation, but he is very soft-spoken in a way that's extremely appropriate for someone trying to keep people calm and focused.
When Cloud pipes up and notes that he's from Nibelheim, Sephiroth comments on it ("Hmm... hometown.") The first thing Sephiroth does when they actually walk into Nibelheim is ask Cloud, "How does it feel? It's your first time back to your hometown in a long time, right? So how does it feel? I wouldn't know because I don't have a hometown."
This is almost certainly not a purely innocent question asked out of pure care for Cloud's feelings. Sephiroth is probably letting himself be a bit selfish here. But if it's a selfish question, it only shows that Sephiroth craves human connection and life outside of Shinra. He wishes he had a hometown to go back to.
Cloud asks about Sephiroth's parents, and Sephiroth responds, "My mother is Jenova. She died right after she gave birth to me. My father..." *laughs* "What does it matter? All right, let's go."
In town, Sephiroth says, "We leave for the reactor at dawn. Make sure you get to sleep early. All we need is one lookout. So you others, get some rest. Oh, and... you may visit your family and friends."
So while Sephiroth is definitely messed up by the fact that he doesn't have a hometown or parents to go home to, he makes a point of telling Cloud that he can go see his mom. In other words, Sephiroth is encouraging Cloud to find happiness in something Sephiroth does not and (to the best of his knowledge) cannot have.
At the Nibelheim inn, Sephiroth reminds Zack that they have an early start tomorrow, and says that he should get some sleep soon. The inn is also one bed short. It appears that Sephiroth stayed up all night so Cloud and Zack could sleep well.
When the bridge falls and one Shinra infantryman goes missing, Tifa points this out. Sephiroth responds "It may sound cruel, but we've no time to search for him. We can't go back, so we must go on. We'll travel together from here."
Now, this here does seem a little cold, but given Sephiroth's prior insistence that everybody else get some sleep, it seems Shinra had given him a schedule that he feels he can't break from. He's clearly aware of how this looks, though, and he tries to reassure Tifa.
During their travels through Mount Nibel, we see that Sephiroth seems to enjoy explaining things to people. Either Cloud or Zack ask what's up with the mysteriously colored cave, and Sephiroth responds, "It must be the Mako energy. This mountain is especially abundant in it. That's why the Mako reactor was built here."
When Cloud asks about the Mako fountain, Sephiroth responds, "A Mako fountain. It's a miracle of nature." Then he explains materia to Tifa: "Materia. When you condense Mako energy, Materia is produced. It's very rare to see Materia in its natural state.
Zack asks, "By the way... Why is it that when you use Materia you can also use magic too?" Sephiroth responds, "You were in SOLDIER and didn't even know that? The knowledge and and wisdom of the Ancients is held in the Materia. Anyone with this knowledge can freely use the powers of the land and the Planet. That knowledge interacts between ourselves and the Planet... calling up magic... or so they say."
Either Cloud or Zack refers to magic as "a mysterious power." Sephiroth laughs and says, "A man once told me never to use unscientific terms like 'mysterious power'! It shouldn't even be called 'magic'! I still remember how angry he was."
Cloud or Zack asks, "Who was that?" and Sephiroth responds, "Hojo of Shinra, Inc... An inexperienced man assigned to take over the work of a great scientist. He was a walking mass of complexes."
I want to note here that Sephiroth grew up around Hojo, and he easily could have internalized Hojo's arrogance. But instead, we see that he recognizes that Hojo is a horrible person who isn't half as smart as he wants everyone to believe, and chooses to answer questions without being a condescending asshole. He does show a little incredulity when Zack asks how materia works, but in fairness, one typically would expect a SOLDIER to know that.
When Zack and Cloud go into the reactor, Sephiroth continues explaining things to Zack. Now keep in mind, he literally just saw the name "Jenova," and this is probably rattling him more than a little. However, he holds it together, and he actually makes an effort to reinforce the information he told him about materia earlier. Sephiroth says, "This is a system that condenses and freezes the Mako energy... that is, when it's working correctly. Now, what does Mako energy become when it's further condensed?" When Zack correctly answers that it becomes materia, Sephiroth stays in educator mode and says, "Right, normally. But Hojo put something else in there. Take a look." So basically, Sephiroth doesn't just tell Zack things and expect him to take his word for it. He encourages him to investigate things himself.
That's nurturing behavior.
When Zack's confused by what he sees, Sephiroth explains "Normal members of SOLDIER are humans that have been showered with Mako. You're different from the others, but still human. But, what are they? They've been exposed to a far higher degree of mako than you." Zack asks, "Is this some kind of monster?", to which Sephiroth responds "Exactly. And it's Hojo of Shinra that produced these monsters. Mutated living organisms produced by Mako energy. That's what these monsters really are."
Zack asks, "Normal members of SOLDIER? You mean you're different?" And that's where Sephiroth starts losing his cool as he spirals into panic over the possibility of being something other than human.
And then as we all know, Sephiroth spends the next several days in deep research. He refuses to talk to Zack as he desperately seeks answers, and by the end he's literally calling Zack a traitor because he's descended from the humans who split off from the Cetra and began living in permanent settlements. And then he goes and burns Nibelheim before going up to the reactor again to retrieve Jenova's head, and you know the rest.
So yeah, the OG game very much did establish that Sephiroth was kind, compassionate, and even nurturing before his breakdown. He cared about Zack and Cloud, and he genuinely wanted to look after them.