Either is fine. My gut check as a native speaker says that in your sentence, was sounds more detached, removed from the action, while got is more immediate.
I say this as someone who has to write about violent trauma as part of their job: I would never use got decapitated in a formal report unless I was quoting a witness who said that, because it's bad enough having to read these things to begin with, the last thing I want is to increase the immediacy of the description for whomever the poor person having to read it is (sometimes myself in the future) and possibly vicariously traumatize them (even worse). I want my text to be as unemotional and sterile as possible, and to leave any emotional impact in the content of the words themselves.
If I'm writing fiction, and I want to increase intensity for the reader, I might very well use got decapitated.
The choice is yours.