If Shredder killed the turtles or visa versa
 

Would either of them actually get in trouble for doing it? The turtles and Splinter aren't people. It might not actually be against any law to kill them. Shredder is a person, but it might not be illegal for walking and talking turtles and a rat to kill someone. Both Shredder and the turtles tried repeatedly against each other to do just that.

May I ask why you keep talking about the legality of this? Shredder is not a character who cares about the law and the law likely does not apply to mutated turtles.

It's like asking "What is the legal status if someone dies in a fight between a gangster and a bear?".

...I've honestly never thought about that.

Anyway, it's not like killing the Turtles would be the only bad thing Shredder would do anyway. So even if, for argument sake, he'd not get in legal trouble for killing off the Turtles and Splinter... he would for all the other shady stuff he does.

I was just trying to look at it from a different perspective. That's all. I don't think people really appreciate how violent the show really is.

It would have been hilarious if at the height of Turtlemania, there was an animated theatrical movie where Shredder grows a spine and wipes out half the cast so Playmates could sell new toys, with a killer soundtrack to boot.