I agree -- I would be totally shocked, honestly, if the stains on his bed are related to the victims / crime.
While he could, as you say, shower and clean after the crime, it would be harder for him to purge any hairs or fibers that might have stuck to him or his clothing after the crime. Even shedding them in his car or apartment could have happened and he probably wouldn't have realized. Maddie, Kaylee, and Xana all had long hair and -- I speak from experience -- it's likely their apartment was covered with hair. The surviving roommates also had very long hair. If he managed to bring some back to his apartment, that would be very incriminating. IMO.