I completely agree with the first half of your post. I think this was an aimless individual meandering and enjoying rambling around and being destructive in the church. A step up from graffiti, but similar mind set. Then we have the added psychological wrinkle of the combat gear.
Everything about this Perp's affect, their movements, and aimlessness suggest someone with limited acuity.
Aside from what has already been dissected and discussed, I find two other aspects of this Perp very intriguing, and will dwell on more:
1. They didn't use a gun (at least that I have heard reported). That is notable. Why not?
2. They were seen on video reorganizing the "tools" they were using from part of their get up. What other profession would commonly do something similar? Have tools on them in this manner, and reorganize them?
I like those two questions. On #1 I believe it is likely -IF Missy wasn't targeted - that SP used what he had to try to incapacitate Missy so he could make his exit without anyone to ID a vehicle or other means of escape. Still I believe the choice of murder weapon is a telling point whether she was targeted or not.
On #2, I can see that point. I don't know if I would use the word 'organize'. But someone who uses tools in a trade such as a carpenter is accustomed to reaching for tools from a tool belt sometimes without even looking. One might be able to make the point that this might be someone who wears a tool belt in their 'day job' and is used to reaching for and returning them just out of habit. Makes me have another purpose to look at the video again.