Disarmed AU
AU where Casey Jr is the reason future Leo lost his arm.
When they first found him he was a kid that'd been living on his own for a few years after he lost his parents and survived only by treating everything as a threat. So of course something as large as an adult Leo prowling around is something he'd be on edge about.
It wasn't even that bad an injury, just a little nick with an old knife. A much smaller Cassandra, who didn't seem nearly as threatening, laughed at Leo's misfortune as he struggled not to swear in front of a child.
Except, an old knife like that is also usually dirty, carrying germs, bacteria, and diseases. With most medicine already not working on the turtles before the apocalypse and now there being a very limited supply It quickly started turning the wrong shade of green and had to be cut off before it spread any further.
Casey Jr doesn't find out until he's older Leo lost his arm because of him. For years he thought Leo lost it in battle with the Krang.
Jr's new mom, who literally picked him up shouting "This little fierce warrior is mine now!", proving she can toss Leo like a salad makes him a lot less scary. Though he does still prefer her being there the first few times, just in case.
Leo never blamed the kid for it, but Casey Jr has seen Leo struggle with phantom pains, having to relearn to fight, and the looks of pity. And it was him that caused all of that.
You have the AUs of Casey Jr taking a while to warm up to Donnie because big, scary, mad scientist. May I offer Donnie taking a while to warm up to Casey Jr because he's the little monster that caused his brother to lose an arm?
Casey Jr asks Leo why Donnie doesn't like him in the most dejected voice. Leo says he just doesn't like a lot of people. Donnie can barely stand him and they're brothers. Leo's lost count of the times he's just walked into the lab and had a gun pointed at his face.
Donnie is how Casey Jr finds out the truth.
