As a few people have already pointed out, he emailed to say he didn't want to carry a gun AFTER Sarah had gone missing. And I believe
@LeopardLeotard said the ambulance was at his house AFTER Sarah went missing. IMO This could point to him creating a narrative of poor mental health AFTER Sarah had been kidnapped and/or killed. It also links to hurting himself in his cell.
The MH issues are all being evidence by things he did after Sarah was taken. Maybe I'm being too quick to assume, but it could easily be him setting up a narrative of being mentally unwell to cover his tracks or so that he would not end up in a regular prison.
I find it interesting that his head injuries were likely self inflicted but actually quite quickly treated and possibly just superficial.
Moo