I think you're applying real world self-defense logic too rigidly to the situation.
In the real world, yes, self-defense would not apply due to his calculated actions after. But in the confinement of the academy? What were they supposed to do with Sayaka? They can't just confine her to her room eternally. What kind of thoughts would be going through Leon's head at the moment? The fear of her (or anyone she might be in cahoots with) making a second attempt on his life? The breakdown of any faith he might be trying to uphold that they can all make it through alive, the paranoia that comes with that, etc. Who's to say he'd even be believed vs Sayaka herself if he just alerted everyone else? She was the one with the bruises of a sustained attack. I find his actions to be rather reasonable within the logic of Monokuma's scheme. He didn't even try it to pin it on anyone else like a normal Ace Attorney villain would. At least not in the moment. Everyone here is a victim, even the people who end up killing, due to the situation they are in. But Leon is moreso a victim than Sayaka. IMO. ---