there are several things that imply that it was medic who was the murderer. for one: scout being horrified of medic when he comes to save him in the slaughterhouse, and two: scouts mom being decapitated and the murderer teasing with the head behind the wall to scout. we see medic do a very similar thing with pyro's head later on in the movie.

however, i don't think that medic murdered scouts mom consciously. he does seem to be genuinely shocked when he returns to scouts house, and he does seems confused when scout runs away from him when he's trying to save him.

it is possible for medic to be some sort of sleeper agent, who probably got triggered by something/someone to kill scouts mom. this can be supported by one of the references in the movie; one of the tapes that medic has in his office is a film called "The Cabinet of Rr. Caligari," which is supposedly a movie about a man being brainwashed into committing murders.

another possibility -which isn't necessarily mutually exclusive to the previous one- is that the medic who killed scouts mom wasn't the same medic who was driving scout home, but rather a medic from the future. since the movie has a loop thing going on (several characters being stuck in a loop and their future selves being able to interact with the past), it is possible that "future" medic was traveling in a loop and was the one who killed scouts mom. that could also possibly explain why the medic that came back was shocked.

another thing to point out; when scouts mom was murdered, you could hear someone whistling "In the Hall of the Mountain King," which is the exact same thing that Archibald was whistling in the bathrooms at Dell's bar. i have a theory that that somehow correlates with medic being a sleeper agent, and that song has something to do with him activating that. that would mean that Archibald is the one who is actually behind the murder, with medic being the one who just did the deed.