Malcolm's not really sure what to say about her mother, so he doesn't say anything. In this particular instance, it's not his job to pry.
"That's good to hear," he replies. Is it though? That means if he goes back home, he'll still have to deal with the fact that Ainsley killed someone. It's almost enough to make him wish to remain here.
He laughs a little as well. "Yeah, I realize that sounds kind of bad. I don't wish for people to be murdered, but solving murders helps me keep my sanity."
It's having that puzzle to figure out, the killer to study, that gives him something else to focus on rather than his own problems.