Over the next several days, you will be hit with all sorts of evidence fragments suggesting one motive or another. Don't believe any one detail. Mr. Holmes has already been described as a loner. Proceed with caution on that. Nearly every shooter gets tagged with that label, because the public is convinced that that's the profile, and people barely acquainted with the gunman parrot it back to every journalist they encounter. The Secret Service report determined that it's usually not true.
My point isn't really about the shooter - about whom I know nothing - but just the general phenomenon. Most people without clear significant others, except for a minority of extreme and likable extroverts, have a "loner" existence.