With that in mind let's look at that flashback again:

After Hiruko stumbles and falls behind, Tsunade gives him a "don't mind" gesture with her hand but her expression is pitying. We don't hear what Jiraiya says but he smirks as he's talking and Orochimaru's expression is blank.

These reactions give me the impression that he wasn't their best friend. Friend, maybe - they don't seem to hate him or anything. But there isn't any closeness either. My guess is that he was in their class at the academy but once they graduated to Genin the three became tight knit with each other, and Hiruko no longer fit with them. Which is honestly fine - I think we've all had friends from school who we stopped talking to once we were no longer forced to be around them 8+ hours a day.

However, due to them becoming distant after graduating, from Hiruko's perspective it likely seemed like he was losing his friends because he wasn't strong enough. Thus he became convinced that he had to get stronger and catch up with them.

And how can he do that? By forming a lifelong obsession and mimicking them of course!

While we know full well that Orochimaru eventually had a secret evil lab and experiments going on, prior to that they had a more normal lab setting. This one was likely public knowledge as we see Anko casually hanging out there, so it's easy to reason that Hiruko would try to emulate both Orochimaru's passion for science and their interest in discovering new jutsu. The fact he specifically worked on experimenting with animals also seems indicative of him copying the Sannin, given each of them has an animal that they closely identify with.

It's as if he thought combining himself with an animal (or animals) would replicate the same strength the Sannin gained from partnering with their summon animals. Notice that a snake is one of the first animals we see him experiment with.

(sidenote: the sheer number of similarities that Hiruko has to Orochimaru really drives home how much of a copycat this character is. However, if we look at it from an in-story standpoint it makes it seem like out of the three Sannin, Hiruko is particularly obsessed with Orochimaru)

Then, after seeing Kakashi come home with Obito's eye, able to use the Sharingan despite being born without it, he gets the idea to steal kekkei genkai in order to gain power.

There are a few unfortunate aspects to this:

While this does later make him powerful, it eats away at him physically (again, much like Orochimaru)

Hiruko sought Kekkei genkai in order to have the same strength as the Sannin, but the Sannin are unique in that - unlike many strong characters in the series - they have no Kekkei genkai. No special eyes or anything, they're just exceptionally good at what they do. While those special abilities would make Hiruko stronger, his strength still wouldn't be the same as theirs. There'd still be that degree of differentiation.

And perhaps worst of all is that at the end of the movie, Kakashi tells Hiruko that he should have turned to his friends, as they would have been there for him, and saved him. Some hallucination ghosts then show up and agree, saying they would have been there for him (well, Jiraiya, Tsunade and Hiruzen do - ghost Orochimaru apparently refused to participate in this bullshit and stays silent)

This is a nice sentiment but...is that really true? It's possible the Sannin might have been willing to talk things through. Maybe they could have helped him. However, I've been studying the Sannin long enough to know that regardless of how much they may have wanted to, they couldn't have saved Hiruko.

Think about it: no matter how much they loved one another, they still weren't able to save each other.