The notion that mass murderers are "loners" in general is really a stereotype that's based mostly on entertainment media.  As a result, anybody who's met a mass murderer but never knew him well is going to "remember" him as a loner without realizing that he/she is just quoting from a cultural narrative.

For example, early on in the investigation of Columbine all the students asked about Harris and Klebold described them as "loners." But as the investigation proceeded, investigators learned that they had a substantial social circle, just not one made up of the Kewl Kidz.

When the Secret Service attempted in the early '00s to come up with a profile for school shooters (they concluded that they couldn't) one of the facts that came out was that fewer than a third of them could realistically be described as loners.

Remember also that anybody can be made to look as if they have serious psychopathology if you simply pull isolated incidents from their lives out of context. Clayton Hartwig, falsely accused of causing the explosion on the USS Iowa, had a "profile" that anyone would recognize as "psycho" unless they knew how it had been put together (or had actually known Hartwig).