lon

In Leo's defense, save the quip in the beginning which is just typical Leo behavior, during the fight with the Foot Clan he is taking it seriously. A moment that stands out to me is when Raph is telling them to retreat and Casey tells them they can't or things will get worse. Leo looks at him, like he's asking how he wants to proceed. Raph, already frustrated with Leo, brushes off the question thinking Leo is just. being Leo. He's not, he's listening, he's taking this seriously. It reminds me a lot of the S2 finale when Splinter thinks he's messing around but he isn't.

Afterwards he proceeds to go off the rails after Raph gets captured because his mental state is really. not great.

Raph got hurt--Krangified. Then he tries to fix it and Donnie and Mikey almost die because of it. Donnie merges with the technodrome, a part of the plan that doesn't even matter in the end, his plan traumatized his brother for no reason. Mikey saves him and gets injured because of it. He'll watch as his brothers struggles with the trauma of that afterwards (I mean Donnie and Mikey's injuries aren't confirmed but c'mon.)

His failures lead to the deaths of thousands in New York, lifelong injuries and trauma to the people he loves the most. Directly because of him, the key is more indirect if yk what I mean. Leo feels guilty for his inability to get things under control, not for losing it in the first place. It was his mistake, his plan, his saving. Things that wouldn't have happened if he was better. His inexperience shown through, which isn't his fault per say, but he has a lot of lessons to learn in leadership. Lessons he did learn, but not quick enough (after all, how could he?).

Leonardo sent Casey Jr, his student that he passed the lessons he learned over the course of his life, to the past. In a sense, he was sending a guiding light to Leo, someone who indirectly knows where he went wrong. Casey Jr was frustrated that Leo wasn't following the lessons his future self taught him, tells him what he needs to hear. Listen to your team, you don't have the answers and that's okay, that doesn't mean you're any less capable. You have a team so everybody's strengths can fill in other's weaknesses. Use them, and understand the weight your decisions carry (the weight of the lives that will be affected by it) and make them carefully. Yeah, it's delivered harshly, but that might have been the only way to get through to him, and 2 oh Casey Jr the poor kid.

I think he knows it's unfair to expect Leo to be Leonardo, but at the time of that scene he was a bit of a mental wreck. in a new timeline, everyone he knows is dead, died in front of him just hours before and he never had a chance to process it. but he's also staring them in the face, a younger face but their face nonetheless. I don't think he was in a great headspace but was well-trained to put that aside until he was out of the field, but it all came rushing out in his frustration. Casey has a more gentle personality most of the time, but the accumulation of stress, I think, led to his outburst (probably for the best, because Leo needed the tough love). casey jr my beloved you went through so much in the span of like. six hours

Leonardo was years older than Leo when he learned those lessons, eventually passed them on. He probably started off the apocalypse learning those things, too, but they didn't know how to stop the Krang, where the key was, etc. They had no way to know how to stop it before it was too late, Leo did, proves that with that knowledge he's able to save everything.

Leonardo isn't weighed down by his mistakes, no. He understands they're inevitable, that all you can do is get back up again. I think more than anything he would have some imposter syndrome. The guy had the entire Resistance on his back (with his team, yeah, but being leader has a certain type of pressure. he's the plan guy. he's signing off on every decision. it's his call), everyone was looking to him, only for his best efforts fail and fail again because of some unforeseen thing.

Knowing the past version of him and his brothers are capable of stopping the invasion would heal the part of him that replays how he could have done better over the years, the part of him that wonders if he was even capable of saving the world in the first place. He did the best he could with the information he had. If he had it all, things may have ended differently. But that was out of his control. It wasn't that he wasn't smart enough, he was missing vital pieces of the puzzle. Leo and their past selves weren't, and they did it.

Beyond that, he would respect the hell out of Leo for learning and growing so quickly. (I like to imagine Leonardo stumbled through the first years of the apocalypse, that Leo managed to get his feet on the ground a lot faster than Leonardo did at his age). You're not supposed to know how to save the world at 16, even if that expectation has been placed upon them. Not only did he know how to stop the Krang, be became the leader they needed to pull it off.

And don't get me wrong, Leonardo is tired from the apocalypse, weighed down by trauma and grief and the horror of everything he bore witness to, decisions he had to make, etc. But he's at peace with the role be played in it. In comparison, Leo ruminates on the mistakes he made all the time, is dealing with his own imposter syndrome. Berates himself every time the scars his brothers have make themselves known, because they're his fault. He didn't know that the Krang would rip Donnie out of the ship, that Mikey would get hurt saving him, these are things completely out of his control. None of them knew, could have known. (Not that Leo thinks his brothers would ever leave him behind if they can help it, he just didn't know they would be able to save him. That Raph was there, that Mikey would open a portal.)

In short to this long post: Leo He hates that his mistakes lead to awful outcomes even if he saved the world. How his decisions affected everyone weighs on him. He looks back with foresight of how he could have one better, much like Leonardo once did. But Leonardo understands that collateral happens, as awful as it is, that mistakes happen. He understands that Leo had a lot to learn and respects that he did. And the part of him that did look back on those mistakes, wonders if he was incapable of the position he was placed into in the first place, was silenced by his 16 year old self, who was able to save the world when he had the information Leonardo lacked at the time.

In terms of the key, Leo also places Leonardo's timeline onto himself. It all ended that way because of that dumb mistake. After seeing the Krang for himself, he knows there's no way they could have managed to defeat them. But Master Leonardo did, helped everyone through it, managed to raise Casey and formulate a plan to save the world. Leo looks at Leonardo and sees everything he isn't. Leonardo looks at Leo and sees everything they've always been capable of.