T.S.
So the teacher didn't say anything to the boy who strangled? That seems odd, she should have said something like, we don't put our hands on our friends, or what you are doing is hurting your friend, something like that. I would ask the teacher about it directly.
As far as the kids in the park, yes that's normal, but again a teachable moment. I hope YOU said something to them, again, something like, we don't hit/kick our friends, or it's not nice to exclude our friends.
As long as adults are continually correcting and guiding children's behavior they WILL learn. That's a BIG part of preschool, learning what is appropriate and how to get along with each other and solve conflicts.