Good post! Let's start with why.
*Argument with GF, wife, mother so he went for a walk to blow off steam and he took his rage out on the girls...
*Revenge kill to send a message to someone.
*Thrill kill
Everyone feel free to add to this list. These are some thoughts that have been bouncing around in my head and all is MOO.
@waltzingmatilda, I like your thinking. Am thinking that, just prior to (i. e., by "prior to", I mean in the previous few month, or months) his Feb. 13 meeting with A & L on the bridge, the POI had a beyond-horrible argument (including breaking up) with his GF (w/whom he had *thought* [operative word here] he'd be spending the rest of his life). Made him explosively furious. Yet he didn't act out his anger (punching on punching bags, kicking holes in drywall, etc.), but instead, "stuffed it", ultimately becoming darkly depressed...
[Keep in mind that this is *entirely* my own theory, although I have been around a few such angry individuals in the past.]
In that state of darkness and despair over the break-up, he perhaps viewed...*advertiser censored* (something he may have *never* done prior to the upsetting argument). Without realizing it, he got addicted, then... on an unexpectedly gorgeous weather day in mid-February, decided to do the unthinkable. Being a quiet, mostly-loner kind of a guy, he went for a walk, but couldn't escape the dark thoughts.
The few who actually *do* know him have *no* idea that he's ever even *looked* at *advertiser censored* ("not *him*", they'd say, if we could ask them), so of course, in their minds he is totally an ***impossible*** suspect.
MOO and utter speculation.
NB: In the past I have become good friends with women who later shared with me that their husbands (who had each been involved in "people"-type work, by the way) had tried to get "unstuck" from viewing *advertiser censored*. (One *did* get unstuck, and renewed his marriage; the other, very sadly, took his own life.) None of their co-workers had any idea what was causing their dark depression and changes in behavior.