Super aggressive tiger barb?

I have a 15 gallon fully cycled tank that I've been trying to stock with some tiger barbs. The first time I stocked it I had 7 of them. They did fine for the first week, they were all about the same size, seemed to be content overall, didn't pick on eachother much. The second week however, there were 4 deaths. At first I thought I had a problem with the water so I ran tests, no ammonia issues, no pH unbalances. So I thought maybe it was just something wrong with some of the fish. I kept an eye out for all the diseases I knew about and they didn't have any. Then about a week later before I went to add more, two of the three fish left were found pretty much shreaded. I've been feeding my fish regularly twice a day, but this one tiger barb I have left has killed every other fish I have tried to put with it, from green tiger barbs to other standard tiger barbs. So now I've got the little thing in a one gallon tank complete with a filter and light for a heater as a form of isolation so he will stop killing all the other fish. He's stayed healthy in the small tank for the last three months and my new tank is now happily stocked with a different shoal of barbs, but I fear putting him in the tank with the others. Any advice on what to do with this little demon? (Killing him is not an option.)