Loomis has always felt himself responsible for the fact that he did not stop Michael when he first murdered his sister, and so he's got that guilt to live with.
In a sense, all actors are character actors, because we're all playing different characters. But a lot of the time - and I don't know, because I'm not a writer - but writers a lot of times write second- and third-tier characters better than they write primary characters. I guess they're more fun.