The justification of quick death vs suffering shouldnt really enter into it. Fish eat fish. Plain and simple. When my rainbow shark started nipping at my Bichirs he went straight to the Oscar tank. That was my rule: Mess with my Bichirs, meet my Oscars. If you have a nuisance and possible danger to your fish you cant wait around for someone else to take it, it needs to go right then. You did the right thing, though I may not have been able to bring myself to cut 'em up. Though I do threaten my fish with the freezer when they act up lol.