Leo starts getting more and more serious due to the constant pressure of being leader, losing his father, and all the stuff that happens around him and his brothers
at some point his plans start getting a lot more dangerous and Leo dependent (If that makes sense) basically he starts trying to do everything on his own because he developed a strong "it's all on me" mentality and now whenever they have to do something dangerous it's like he tries to take the whole fight on his own to make sure it's quick and won't become a huge situation, at some point he resorted to just killing tye source of the issue which shocks and worries his brothers
In this image they're in space again bc of another situation, Donnie found a way to end it but it's very dangerous and could lead to death, he tells this to his brothers but Leo is like "let me handle this" and Donnie is telling him how dangerous and stupid it would be to do that but he's not listening to him and rejects all tha he's saying
It's during this scene that the brothers (raph) finally have enough and start an intervention (aka raph starts yelling at him and telling him how stupid he is) but even after that Leo finds a way to get through them and finish the mission, the brothers tried to stop him but faild
Whether Leo survives and heals or he dies with the "it's up to me" mentality is up to you (. ❛ ᴗ ❛.)