My villain is the Solicitor General of the United States, who leads an effort to murder witnesses in the Witness Protection Program - crooks who got off and didn't have to pay for their crimes because they testified against bigger fish. He gets a list of the most "egregious" ones - the criminals who did the worst deeds but got let go, placed in the Witness Protection Program due to their cooperation and testimony. He has two henchmen that work for him and these witnesses start getting murdered randomly across the country.
This is plausible, in a ridiculous but historical manner - when the Witness Protection Program started in the late 1960's, there were plenty of Department of Justice leadership folks that were against the principle of it (rewarding significant crooks who tell on their bosses). So, I can mention that. But it's also highly implausible, as someone like the Solicitor General of the United States ain't gunna be someone going around murdering people.
So, I have felt like a backstory is indeed necessary. Just jumping in with "Meet your Solicitor General - People's Advocate and criminal mastermind" probably isn't going to work. I could do a Batman-esque thing, where his long lost uncle was murdered by someone who then escaped justice through the Witness Protection Program. But that's too tropey for me. So, I went with a legal, Supreme Court type of justification, finding an actual SCOTUS court case from back in the 1980's that delved into these issues. In that case the defendant was held accountable - he wasn't allowed to "escape" his sentence. My villain is placed as a young SCOTUS court clerk who works on that case, forever shaping how he feels about it. He's also very religious and comes at it from that angle as well - God's avenger.
On and on.... Back to your point, I feel - in the end - that I have almost spent too much time on it, and also haven't built a justification other than a very thin one. It works, but just barely. My concern is the word count spent on it. Around 1.5 chapters, maybe closer to 2 (novel is full-length - 86 chapters).
Anyway, I wrestle with this issue. Any thoughts?