Personally I think OHKs should be possible with certain weapons under certain pretenses.
Like if a grenade goes off at your feet or a molotov hits you dead center mass that should kill you. Seems unfair yeah but if you're standing around long enough for a grenade to 1. land under you and 2. have time to detonate, you deserve to lose that one. And on a game like Brink you really shouldn't be doing too much standing around. Also, headshots with certain powerful weapons or a headshot point blank with "oomphy" guns (not as powerful as a short rifle but like a shotty or a .45) should be an OHK. Once again, if you're hanging around long enough for an enemy to get that close to you and line up a good shot before you notice him and move or retaliate, you deserve to die. Granted I'd be ticked off if somebody did that to me but also when you got an OHK on someone it'd give you a feeling of power. Especially on Brink, where OHKs would be pretty rare. Plus if I did get killed like that, hell it was my fault for not keeping in motion.