Using the kill command should only be done if two invis races are stuck and are the last two alive. The one ego should use it is determined by the type of map: t on de_ maps, CT on cs_ maps. Not doing so is in essence excessive camping. Using kill to deny someone a kill is against the rules in my book. It's like you're losing in checkers and you just throw the board across the room. It's very poor sportsmanship, and shouldn't be encouraged.