I'd say that that scenario, worked out better than 99.9% of any similar scenarios.
In my state, its also illegal to trespass, its also illegal to tag a deer you didn't legally kill. In this case, the first shooters had no claim, as the landowner legally killed the deer.
Here's how it works on OUR property. Someone shoots and wounds a deer severely, it comes on to our property and I dispatch it. If the hunter who originally shot the deer does come by tracking it, I'd give him his deer. I just merely stopped it from running any further or worse.
In the case you presented, I'd have given the deer to the youngster.