It's simple, simply allocate points for doing team objectives, and here's the best bit: no points for frags. Sort scoreboard by points and that should solve everything.
The system should balance itself out neatly, those who care about points will do that just that, score as much points as they can, those that only care about k/d will then counter balance those that only go for points and then pretty much both sides will be shooting at each other and trying to get objectives done.