Two weeks left in 2020 - statistically, there's a 4% chance the biggest event of 2020 has not happened yet. (Or perhaps it's a bit higher than 4% this year …)
Most HAM radio repeaters are operated for the community by either amateur radio clubs, or radio enthusiasts who want to make a resource available. There are likely not "bad repeaters" just community resources used by bad people.
Repeaters are most often used for communications over say 5-100 miles depending on terrain. Most short range HAM communication is simplex (direct radio to radio, not through a repeater) in a way analogous to the low power FRS radios available in WalMart etc. Longer distance communications may also be direct, using more exotic frequencies (shortwave for example), this may be what Nellie Orr was doing (don't recall specifically, but likely)
As for decoding - most HAM comms are just voice on the open channel, no decoding needed. There is some packet comms that are an amateur equivalent of internet, that's not as easily accessible.
If the bad person has a HAM license, they can just go on air and talk, like anyone else. (Similar to how anyone good or bad can write a message here.)
If they don't want personally go on air at a certain time, they can ask someone else to do it.
With repeaters, the radios that transmit through the repeater are generally hand-held walkie talkies, as they're intended to pick up weaker signals and relay them. Easy to operate.
Of course. Also a hamanon here. Looking to help those unfamiliar with the subject get oriented and realize repeaters are not their enemy (like 8kun is not their enemy).
And for anyone out there who is unfamiliar with ham radio, it's a super-essential independent backbone for communications if everything else goes dark. Preppers love it.