With rapid fire, you could kill anyone EXTREMELY fast even if it did just a quarter of a heart of damage..
Honestly if this does get added it should be somewhere like 1k points, and it should break relatively quickly.
If it does 0.5 damage, it'd cost (to take 10 hearts out), 80 points for the arrows necessary. If it was 1, it'd cost 40 points per person, with a net loss of 30 each kill, and slowly less. Not counting you'd have to be standing moderately close to people or you'll probably not hit them, I would say it's not broken. Here's to respond to some problems I've thought of and solved:
Shooting in Crowds randomly - This would not work well, considering the amount of damage done, you mine as well just throw grenade and do slightly more.
Getting people caught in corners - You can presumably place TNT behind the person and kill them, even if they kill you. Also, its pretty hard to get stuck in corners, at least from what I've played. You could also you use a knockback sword to get them away.
Another thing is to get them away with the knockback sword like above. It'd basically be a counter to it- any knockback sword. You could knock them back and place TNT and if they try running back, they die, or have to stand back and avoid it, giving you time to get away if need be.
Sorry if the post is a bit long, just wanted to answer some things I thought may come up.