My best guess is that Jo caught him in her apartment. He was "bored" all right. The girlfriend at a party. Why not amuse himself, poke around a little?
Was he in the bedroom..maybe looking through her lingerie...and she let herself in, and started to settle in before realizing it? Do we know where the old connecting door was?
This would be something so embarrassing to him...how to explain to his girlfriend, his work colleagues, his family? The amazingly high opinion they all held of him would be shattered. He would be.."a weird guy."
IMO Tabak could not deal with that sort of personal mortification. I think he would kill to stop it.
I'm not sure he will tell the truth at trial. I suspect he will try to make himself "a victim."
But, if there was no previous violence, and no relationship between the two, I think he murdered Jo to cover up something that would, in his mind, destroy HIM.