You should give points for damage. I suggest that for each point of damage, a player gets 0.001 score points. So if they killed somebody entirely by themselves, they get 0.1 points, which is 10% of what a kill is worth. If this is implemented, players will get points more fairly and their kills won't be "stolen".
I've made a script for damage rewarding in my own server, and if you want, I'd be happy to make one for your server, and with adjustable score rewards for each point of damage.