This just happened not too long ago in lvl 7x Abyss:
I got in, saw a lvl 6x fighter and a lvl 7x archer fighting, behind the level 6x fighter is a lvl 5x cleric healing him...
I watched the fight for a good 5 mins, the archer took 2 unexpected shots and killed the cleric, while continously potting (he must have got cash shop pots)
So the target changed, the archer trying to kill the cleric, cleric continously healing herself, and behind the archer, the fighter was hitting him... like this:
And yeah i just stood and watch.... trying to find out if its was friendly fire..
after a while i decided its not, so i partied the archer (who's lvl 78) and knocked them both down...
Fighter and archer left... then two relatively high level (70-80) mage and archer appear.. and started trying to kill me / my archer... My archer killed the other archer... but didnt/couldnt take down the mage.. so we opted out of abyss LOL
and we came out and they said to the archer "you're gonna get warred" and the archer explained that it was the other person that KSed first...
hmm i ran away not wanting to get involved more.... LOL!