KOS is an abbreviation for Kill On Sight (self-explanatory).
There was an uproar on the PvP server Darktide (AC1) back in 2000, when players found out that on the other servers (that had optional PvP) people used the expression KOS and L, for Kill on Sight and Loot.
"What the hell? Does it mean that normally they don't loot? They kill someone AND DON'T LOOT HIM?!? But... ".
Suffice to say that Darktiders were absolutely appalled by a behaviour that violated so blatantly the professional deontology of any self-respecting PK. I'll have to get D34dly to hear the expression in a future episode. He'd probably faint.