I've seen cases where older men suddenly turn violent, but their motivation isn't so much a psychological tendency towards violence for it's own sake, but IMO a determined and concious decision to be ruthless, because they realize that's the only way they'll get away with it.
For eg, a recent case of a perfectly law abiding man in his 50s, Paul Caneiro, who, with premeditation, shot and stabbed his brother and family (2 children), for money.
Al Blanco, in his late 50's with no criminal record, with premeditation lured, shot and buried his "friend" and employer Sid Cranston, for money.
CM has a long record, going by this article. He didn't clean up his act and get scared straight, in spite of repeated jail time, over several decades.
McStay murder mystery: Who is Chase Merritt?
Even though he was repeatedly caught and sent to jail, he continued to commit crimes to steal money. IMO, he may have had enough of getting caught and decided the solution was to be ruthless. ETA, also possibly by then, unlike earlier in his life, he would perceive he had a lot to lose by getting caught