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.
What about a percentage as someone suggested earlier...staggered to increase as the WZ nears completion - large balance (51%) awarded upon completion?