Simmons (the woman who murdered the guy) was not the one he had the relationship with. Her trauma (as described in the article) was that she believed her stepfather (whom she stabbed 5 years ago) abused her daughter. As far as the article covers, she had no involvement with the guy she shot and dismembered at all. He had a warrant for failing to register as a sex offender; Simmons apparently saw that and, because of her "disdain for pedophiles", murdered him.
Whatever your feelings on the guy, we simply can't condone extrajudicial killings. I know it feels good sometimes to think "Yeah, that guy deserved it!", and in some cases, maybe they do, but it doesn't matter - it's still vigilantism.
In this case, it seems that she's just a violent woman. If I'm reading it right, she stabbed one person, and murdered two (three separate unrelated incidents) so she deserves the life sentence she got, however you slice it.
There's always a relevant XKCD.