You just need to get more kills than deaths in any game. It accumulates: if you get 10 kills and 0 deaths in one game, the bounty will still mark as 10. Getting 2 more kills than deaths in the next game, for example, will net you 12. Repeat until you have 25. Did this in about 3 games; it's only hard if you're not good in the crucible. I think you scrapped a perfectly good bounty.

what would it of given me if i had completed it? just curios.