Personally, I think I could kill someone. I've never really hurt anyone really bad, but if I hated someone with enough passion, killing them wouldn't be a problem at all. I'd make it simple, ambush them and gun them down. maybe I'd say something to them or let them see me before I shoot them. An execution would be nice too, one shot to the head or multiple to the chest. I never did like stabbings, it just let's them suffer before they die and most people don't deserve that.