That's only because their aggressive by nature, and their not social fish and shouldn't be put with other fish who they might think would take over their territory. I'm sure he didn't want to hurt/kill your other fish it was just territorial instincts . Just try putting your Betta's in separate tanks from other fish. One Betta per tank. ;) Their really fun fish See More▼ to have when they are separate from other fish. :D