Why do WC's crimes appear to have escalated so very suddenly and seriously? Why, as an experienced police officer, does he appear to have made such a clumsy and half-baked effort to evade detection?
I'm not sure if WC knew he'd been reported for the McDonald's drive-thru flashing incident, but it seems probable given what we know (two police were called to McDonald's, and there's an inquiry going on that suggests they knew WC was responsible from the off). But either way, from that day on he would have known his number was up: it would only have been a matter of time before his police career was brought to an end in deeply shameful circumstances.
We know his life was already spiralling out of control (the long periods of unexplained personal leave, talk of 'erratic behaviour' etc). I agree that his build in recent photos looks very much that of a steroid user (right down to his round 'moon face'), with all the behavioural implications. This all starts to look like a very (VERY) dramatic runaway midlife crisis.
WC knew he was falling apart, and that he was about to be sacked and disgraced. Joining the police appears to have been the crowning and proudest achievement of his life. There is no way of making this sound hurtfully trite, but I think he just considered the situation and felt he had nothing to lose in fulfilling his very darkest fantasy. His own life was effectively over anyway. Might as well be hung for a sheep as a lamb.
RIP Sarah.