Compared to his first appearance in Ayakashi: Samurai Horror Tales, the Medicine Man seems to be more emotionally detached. But when you remember the nature of his job (rooting out a Mononoke and figure out what caused its birth), it makes more sense. All Mononoke are spirits that are born when people commit some kind of atrocity such as spousal abuse, or murder. Since the Medicine Man is possibly immortal, he would've seen a lot of stories that would make him believe that humans being nasty or at least morally ambiguous is normal, and has since grown used to it.
In the Nue arc, if you think over the events so far, after the reveal that everyone in the house aside from the Medicine Man is long dead, you'll realise that everything the Medicine Man did was done by himself in a house where countless people had died, with that thing only a few strides away.