He came armed. He planned on using lethal force if need be.
Llewelyn surmised from the drugs and the corpses that a deal had gone sour. He tracked the remaining party from the trail of blood.
He took mercy on the victim in the car because he was suffering; Llewelyn had a conscience. But he also had enough curiosity to take the rest of his afternoon to track the remaining party. Judging from the amount of dope, there would be a high reward at the end of the blood trail.
He also surmised that the remaining party (only one man he learned) was wounded, and so waited him out from afar. When he witnessed no movement over a long period of time, he felt safe enough to approach.
If the remaining party were instead multiple men, he'd have a decision to make. Likely he would have tracked them until he found an opportunity to separate them from the money, or until they let natural selection whittle the party to one.
With only one healthy party left, he'd have to weigh his conscience. He wasn't an evil man, as we witnessed. But on the other hand, this is likely more money than he's ever seen. I wager he would have tracked a single man until he could have disarmed him and then taken the money.