What do I do?

I have a black hearted tetra, they are SUPPOSED to be schooling fish from what I was told, this guy ATE all of his fellow black hearted tetra buddies, then ate a beta, then a catfish, then more fish. This guy ALSO killed off a invasive snail species we had for FOUR years, that WE couldn't kill off (we tried everything, from deep scrubbing and replacing the rocks, to boiling everything, nothing worked) but within two months of him being in the tank, they were none existant... Now, my concern is that he looks lonely, he has LOTS of places to swim around, hide, ect, but he picks a spot and stays there, sometimes he'll swim around, and if there's food he's zooming, but this guy has a 10 gallon tank all to himself and he just likes to stare, I feel he is lonely but due to his MURDEROUS history, I'm not too sure it would be smart to get him a "friend" (more like treat :/) so what do I do? I do try to 'play' with him (I wave my finger/hand at him and he does react by coming closer and just more staring, but at my finger/hand sooooo) but I can't do that for hours on end ahah

Any suggestions?