It's certainly much more difficult when you're working in a daycare, as you don't always have a full picture of the homelife, and you don't have all the time you might otherwise have with a child. I'm also aware that not many daycares have an abundance of resources (including employees), so while these are my thoughts, I'm aware that they may not have been entirely applicable in your situation.

For the hitter/namecaller, I'd definitely separate them from the group. If they can't be kind to the other children, they would not be allowed to be around the other children. I'd pull them aside and try to ask them why they wanted the other children to feel bad? And if they themselves were okay? They might not want to open up, and that's fine, trust has to be earned.
I'd keep them with me and ask if they wanted to be my "special helper" (because all kids like feeling special) and give them opportunities to help out with some chores around the facility, especially those that would help prepare for something the other kids liked, to give an opportunity for them to feel responsible for making kids feel better, instead of feeling worse and make sure they knew that the other kids were happy because of them, in an attempt to associate a good feeling to positive behavior.

As for the disruptive behavior, that can be a little more tricky. I'm not a developmental specialist, but my first few thoughts would be they may be acting out to receive attention they may be lacking elsewhere, or might possibly be undiagnosed adhd.