Based on the episode from TMNT Fast Forward, "Timing is Everything." If you haven't seen that, this isn't going to make much sense.
If you're minded to read anyway, a quick summary: in the episode in question, time becomes unstable (due to Bad Guy Plot Misfiring) and holes are ripped through its fabric, and Leo and Cody find themselves accidentally sucked back into Leo's own past just after they defeated Shredder for the first time. So I had to wonder just what might have gone through Leo's mind at this point...
Standard disclaimer goes here!
The Grandfather Paradox
It's only a split second, but Leonardo maps for himself the choice that he could make.
Oroku Saki is there before him, wrenching free of the ruined water silo, and in another moment he will attack. It doesn't matter that the Leo before him is more than a year distanced from the younger, softer Leo that would at this point be on the roof above, struggling to his feet. Shredder won't notice the difference; he won't take into account that his swords and accessories are different, that Leo has scars that he didn't before, that his very attitude has changed.
Because time travel, accidental or otherwise, is not the first explanation that would ever leap to anyone's mind. And so Leonardo lifts his head high, sword at the defensive, mindful of the vulnerable boy at his back. And he faces the Shredder like he has many times before, and listens to his enemy's voice, so cruel and full of confidence.
"Leonardo. You should have run while you had the chance."
A split second.
Leonardo is much stronger than he was, as are all of his brothers. He has trained exhaustively under two strict Masters – six in fact, if one counted the Tribunal - has conquered every hardship thrown at him. He fought with increasing skill in every confrontation with this worthy enemy, and knows that he is now, at the height of his training, more than a match. He is so much better than he was. And importantly, he knows now – unlike his softer, younger self – that Oroku Saki is not human. He knows where to strike.
And Shredder will completely underestimate him, because Shredder thinks he is someone else.
The temptation is maddening.
He could finish this, surely, in a matter of minutes. Fight Saki, take his armor apart, run his katana through the alien creature within. Finish it once and for all, before Saki ever has the chance to gather his forces and retaliate, months from now.
Leonardo could save himself the thrashing on the rooftops and the scars it left on him, both physical and psychological. April's store would never be destroyed. His brothers would be saved the heartache of Leo's spectacular failure. Karai's promise of peace between their two clans would hold weight, because Karai would not have to make a choice. They would never have to board Shredder's starship; never have to suffer such horrendous injuries. She would never have betrayed his trust. He would never have alienated his family with his anger. She would never have destroyed their home and tried to murder them all.
He could change it. Wipe the Shredder from their lives in one clean attack here and now. Spare them all the madness that would follow.
A split second. That's all it takes. Then the temptation is gone.
He doesn't need Don to tell him. Destroy the Shredder now and Leo destroys himself. Defining acts and deeds that make him who he is, all gone in a heartbeat. He could be an entirely different person … maybe one not equal to the task in front of him now, if his radical changing of history even led to the same point in time. And, after all, no guarantee that what would follow after Shredder's death would be any better.
I am what life has made me, he thinks. And reaches a hand back to steady Cody, increasingly nervous behind him.
He isn't here to change history, but rather protect the future. And so he fights, careful, defensive, putting himself between Shredder and the boy at all times. All he needs to do here is keep Cody safe until Don can find a way to get them home. Don will eventually, Leo knows. And until that point, he will tread carefully to safeguard history, leaving no permanent injury on the men he fights. Leaving as little a mark as possible. Leaving only the memory of his careful, focused skill in the face of Shredder's forces.
An omen, burned into the mind of his hated enemy; that in the not so distant future, Leonardo will be a force to be reckoned with.