"But was it a planned vandalism/burglary gone bad OR a planned murder?"
This is a great point that keeps being missed. There is plenty of evidence for planning - but that planning doesn't point either way, to tell us what was being planned. It just tells us that SP was there on purpose, and didn't want to be caught, so he did what he could to mask his identity. And apparently he did a great job at it, unfortunately.
Yes, I still lean toward something else being planned and Missy walked in on it. If that is the case, SP may have struck her simply to render her unconscious and hit her too hard. This business of striking someone and having them fall unconscious happens in the movies but not always in real life. As I said, and I believe you agree, we have points for both scenarios. Just my opinion, but I believe it was not a planned killing. If it were, the killer could have used a small caliber gun. I see the point that using the tools to be up close because this was personal. Someone that does that would seem to have a higher degree of confidence in their close quarters skills. And if it was personal, wouldn't the killer want Missy to see their face? (Of course, since the murder was apparently not caught on video, the point can be made that we don't know what the killer did or said in the final moments.)
I do believe it was planned and SP KNEW they would be caught on video. But they may have simply planned to leave damage behind and derive some delight from having LE view the footage and see someone dressed as LE. Some sort of strange sense of humor. (If this is the case, it may not have anything to do with that particular church, but directed at local LE.) As far as planning, I keep going back to how much was public about the malfunctioning outside cameras. Did the church warn members about leaving items in their cars since the cameras had not been fixed? Was it in the Sunday service bulletin? If not, we might significantly narrow down the suspect pool.
BUT if this killing was not planned, the killer is having to live with the consequences.