The assumption is that the mercenary does for pay what the soldier does out of patriotism. If both are doing good, then the mercenary accrues no merit before God (or good karma if you prefer) while the soldier accrues merit. If both are doing bad, then then both lose merit because the deeds are inherently wrong regardless of the motivation. "Yes I killed all those innocent children to save my country". "Yes I killed all those innocent children to put bread on my table." Hmmm. Seems to me like people that dislike mercenaries probably also dislike profit making institutions.

Quit being silly. You hire mercaneries either becasue your own soldiers can't do the job or you don't have enough of them. Do you honestly think that mercenaries are any different than police? You pay them both a salary to do a job. Their own motives for doing the job are exactly that, their motives not their employers.