The only companion I've killed deliberately was Minthara. But now that I've had a chance to recruit her with the knock out method and talk to her a bit (after a second attempt, since she bugged out the first time), she is my favourite. So it is unlikely to happen again.
Jaheira, on my first run. I did not recruit her when it was possible, thinking I'd have opportunity later, and she died in the battle for moonrise towers because Larian did not give her any armor. Since I've had a full party, I did not reload. I actually forgot about Minsc on that playthrough, so I guess he lived.
Lae'zel, because my character failed the roll to convince the tieflings to let her out; Lae'zel then asked her to kill them instead, and since I was playing a tiefling, the answer was no.
