You should probably keep it by itself, it's going to keep killing your fish. Remove the dead ones or your ammonia will go thru the roof (if it isn't allready - you should test it and probably do a waterchange)
I don't know if he's killing them or the ammonia is killing them. He is going to need a BIG tank when he grows up!
This link may be helpful
http://animal-world.com/encyclo/fres...erLionfish.php