I don't know why it matters what the handyman was thinking or what delusions he may have had, or if there really was an affair, or it was just his fantasy. It doesn't excuse the crime, he's going to get the book thrown at him, as he should, and this poor woman was brutally killed by someone she knew.
Knowing his thoughts and all of that may not be something we ever learn. The story he's telling is they got in an argument, it got very heated, and he killed her (butchered her). As long as he's put away for life, with no chance for parole, ever, he won't be able to attack another homeowner.