What you're talking about son, IS MURDER!
I don't think the choice of who gets to live and who dies should be limited by what the developers or the software restraints itself with. IMO every npc and character should be killable. How you go about that is entirerly different. I'm not at all in favor of 'friendly fire'. Nothing would disapoint me more if a stray fireball that I hurl accidently hits my paladin in the back and kills him (granted if I could hurl fireballs and my companion was a paladin) But I don't think it should be something as simple as a dialogue option either, like "I don't like your views on the current crisis, my compadre. WATCH ME TUMBLE!"
Personally I would prefere the option to kill one of you're companions be a lot more meticulous, more machiavellian. So instead of just clobering them over the head in cold blood, you could, instead, have the option to stir up one companion against another and hope that if they got in a fight the one you dislike would die or be wounded enough for you to easily finish them of and brand it as punishment so it was more or less over looked by your other companions.
Or if one of your party member has a lot of one liners that were really annoying, like a few people pointed out before, you wouldn't necessarily have to kill them (and lose a valuable companion). Instead you could hatch a much more devious plan and say, "hey i found this needle and thread in my last quest exploit, i'll poison the b**** with my sleeping poiton from earlier and then sow her lips shut" ...no more one liners! (the obvious consequence being that if magic required verbal incantations you would probably put this character out of action.) Way more satisfying than just wacking their head in.
That all being said though, if you did make your companions killable, I think you would need a lot more than 5 companions avalable in game, the loses would out way the gains far to much when your only looking at a miniscuel 5 options, one of each class (ie, warrior, mage etc).