Now im not exactly 100% sure how this happened, but i do know this is not working as intended.
As stated, you have to kill 10 flag carriers before they leave the room.
That basically means, 10 people have to get the flag and you have to kill them right?
Well 3 of us decided to stay back on D and one came and we killed him before he left, then around 8~9 people came (not at once but in pairs of 2's and stuff) and we killed them, then another guy came got the flag and we killed him and we got the achievement.
So basically in a nut-shell I think you need to start by killing a flag carrier, then 8 guys, then kill another flag carrier and you get the achievement.
Its not a THEORY.