I think #3 is way too illogical, because
a there's nothing realistic in "I saw her, so I decided I would kill her"
b the random observer who crossed her path would have no way to know her name, her schedule, etc that puts her at the church that day
c it would require that random observer putting together a costume, getting up in the middle of the night, braving bad weather, and doing it all for nothing
Taken together, I think the odds become so extreme that we can rule that one out. And therefore if there's a randomness to this, it has to flow from the setting (ie the perp happened to be there for another reason, and MB wandered along, wrong place wrong time).
It's not my leading theory (robbery is), but it's my backup. Let me speak to your points, because you seem to be thinking random observer while the way this would make sense would be if the person is a serial killer:
(A) I saw her, so I decided I would kill her... Pretty much every murder of a stranger happens similarly. One scenario I've mentioned before: she stops in to an all night store between home and the church, chats with the cashier about her CG class, and a customer overhears. He either got enough info that way or else he follows her at a distance to see where she goes.
(B) the observer begins to stalk MB in order to find out those things you mentioned. He was a random observer at the beginning of (A), but the crossing of paths stops being random once he starts stalking.
(C) this person, after stalking, is going to take precautions. He will know there are cameras, hence the suit. He will know police don't patrol as often in the rain, so he may have chosen that night purposely. And he isn't doing it "all for nothing" - he's getting the thrill of the kill.
With all that said, like you, I subscribe to Occam's Razor. I think it was a botched robbery. There are probably 100 serial killers active in the US right now which would make it unlikely that one of those 100 happens to be MB's killer. Still, there ARE 100, and they are out there stalking and killing someone. And the longer police are stymied, I think the less likely it will be that it was someone she knew, and the more likely it will be that it was a stranger, either via robbery or thrill kill.
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