Hopefully someone will jump in with specifics on md tress passing laws.
Generally its a minor offense to tresspass on unposted property. If its an offense at all.

If a violation occurs while tresspassing then adds to the penalty.
You stated person shot the deer day after the season?
You are generally allowed to take a kill shot to put the animal out of its misery.
But don't know if day after the shot would count as such.
If game violation did occur then that occuring while tresspassing would increase the penalty.

Without knowing details, gut feeling tells me it wasn't a mercy kill, and he was poaching.
Either way, you cannot enforce that law.
You should let local authorities know what's going on.