Bully

I am terribly afraid my child is going to be a bully.  I know she is only 2 and she is still trying to push boundaries, and learn what is acceptable behavior, but yesterday just scared me.  She was playing with the neighbor in our little plastic pool.  She kept splashing the girl even though I told her to stop and the girl asked her to stop.  She thought it was funny that she made the girl cry. I was so embarrassed and furious that my child was acting like that.  I pulled her aside and explained to her how she was hurting someone's feelings, etc.  She said she understood, but went right back to splashing.  Then at one point she had the girl in a headlock. The girl did push her back and Hannah thought it was funny.  She thinks it is funny to wrestle and push.  Luckily the mom of the other girl didn't seem to mind that Hannah was being a bully.  She kept telling me they are 2 and this is what they do.  I understand 2 year olds can be like this, but I really hope it stops.  I am at a loss on how to explain correct behavior.  I have used various forms of discipline when she is mean, and I try to show her what good behavior is through my actions.  I am hoping she was just extra snotty because she hadn't napped.  I really don't want to have the kid that no other kid will play with.