Keep in mind it's based off the percentage of damage dealt. When targeting a 250 credit infantry with a rifle infantry, you may only deal 25% damage before dying, right? Using my system, that would give you 31.25 credits. Engineers get like 8-10 credits per second repairing a terminal in a building right now, don't they? What does a rifle infantry get if he knocks off half an enemy soldier's hp bar? Like 2 or 3 credits? The rifle infantry is frantically fighting while the engineer is just sitting still holding his left click!
At any rate, you can modify the numbers and treat each basic infantry and even target uniquely to fine tune. The point here is, in a situation like this, basic infantry (which is essentially all you have access to) do not earn enough credits.
Edit: On second though, I'd argue rifles (as an example) should earn more than an engineer because it encourages aggressive play. As it is now, everyone is rewarded much more for being an engineer and repairing buildings, which in my original situation is usually the best play, which promotes hard turtling.