I agree, it is shameful that people in a "free" society would even consider turning in neighbors for this kind of BS. I'd add a few possible motivations (recapping your list too):
Fear. Fear that if they don't, someone else will report their neighbors "violations" and that these people obviously knew about it, making them an accomplice. The left has completely cowed these people into obedience.
Virtue signalling. Wanting to belong, wanting to be a part of the side the leftists and their MSM lapdogs have so thoroughly (but erroneously) portrayed as the good guys.
Brainwashed. They may have so completely bought into the BS that they believe they are doing the right thing. These are truly dangerous useful idiots. These are the people that turned over neighbors to the SS, Stasi, etc. with a clear conscious, believing they were doing the right thing for the greater good.
Dickheads. Some people simply like being mean to others. They are small, petty people who need to do things like that to give themselves an artificial sense of power. They are timid and weak, and hide that by reporting mask violations, or neighbors washing cars or watering lawns or other HOA level BS.
We all know what the answer is to fellow citizens that side with a tyrannical state. Few of us are to the point where we'll actually voice it out loud. Even fewer, if any are to the point where they actually take action against these petty tyrants. Assign it to fear, complacency, self preservation, or what-not frustration and anger are building. When that dam breaks, it not going to be pretty, especially for the liberal Karen's of the country that think their virtue signaling and the government are going to protect them from wrathful neighbors.
I like your peckerhead synopsis. I suspect that lot has THRIVED over the past 12 months.,,on both sides of the aisle. Mask Karens are reprehensible, but I admit I cringe when I hear about some massless John Galt-wannabe intentionally walking into a store where BY LAW you have to be masked and calling everyone “sheep”...dude, respect the private property rights of the store owner, buy your own dirt and preach the gospel there.