I will keep it simple
Now:
2 points/kill, 5 xp
20 points bonus for winning, 120 xp
my suggestion:
kill 1-2: 2 points/kill, 5 xp
kill 3-5: 5 points/kill, 10 xp
kill >5: 10 points/kill, 15 xp
If you win with less than 3 kills, you get the 20 points, if you win with more than 2, you get 30-50...