I also try not to use that phrase, but when I do it means I'm REALLY ticked off at someone. I have a bad temper, and literally see red when I'm very angry. That's when I'll say "I could kill so-and-so". Usually, if someone is just being stupid and annoying I say "So-and-so needs to be smacked with a smart stick".
I'm not sure I could kill anyone, but I can imagine doing it to protect myself, my family, or my friend's children. And, if anyone EVER hurt Ivo I'd put them in a whole world of hurt.