I 2nd that vote. XP for any kill should fix the problem. Then they'll be forced to hit 50. But I"m sure there will be one nut-job who will find a way to bring people down to low health, and let someone else kill them. Who gets the xp then lol. Just a thought.
The game awards kills to pretty much anyone you sneeze near during a warzone. It's why healers and AOE/DoT based classes get the most kills generally. It also rewards players who leave the warzone, which is the primary reason that XP is awarded at the end currently.
I'm not saying your solution couldn't work, those are just the negatives involved with it.