He might know that because he searched the www for head injuries (that's in evidence), it's my conjecture he was looking for a way to do it which would minimize mess. I'm not trying to sale this as proof, or evidence, it's just pure speculation on my part. I think he was looking for an easy, non-messy way to clunk her on the head so that she could not fight back and it would be easier to strangle her, but something went wrong and he lost it, and wailed on her.
Just my conjecture and opinion. AFAIK, it's not evidence, or even testified to.