What type of fish is this?

I was hoping someone could help me identify this fish. It was given to me by a friend with his 30 Gal tank, but he didn't know what it was.

It's about 2 in long and in 1 week, has killed 4 Neons, 2 Red Platys and a 3 in Spotted Cat Fish that I introduced before I knew he was aggressive.

I'd like to introduce some fish, but I don't know what would be compatable.

Any advise would be appreceated.

Thanks.