Probably a psychopath though, which means he will only be focussing on himself and how badly his life will be affected by this. He may not be able to withstand his public image, amongst family and friends, being tarnished by the killing. But he won't care about the victim as he will have told himself she deserved it and even caused it, and he won't be suffering guilt or remorse.
His behaviour towards her has probably involved a lengthy period of coercive, controlling behaviour and abuse, either mental, physical or both, so in many ways, he will have been mentally prepared for harming her. I found the video of the victim in the car being interviewed by police quite telling, she was clearly terrified and upset, and Laundrie had managed to convince the police officers that she was the one in the wrong.
Most psychopaths don't kill. They do do things such as abuse their victims mentally and physically, but most are restrained by the likliehood of spending the rest of their lives in prison from doing serious crimes unless something happens to trigger a loss of control. Some then lose control, and of those, some kill again. But you shouldn't assume that only master criminals kill, most are carried out by husbands, partner or boyfriends of the victim, and only rarely by female psychopaths.
I'd be interested to discover whether Laundrie had a record of petty crime as a juvenile, or at least of truanting from school or of being a poor student, and whether he had a failure to hold down the same job for a reasonably lengthy period. Psychopaths who can hold down a job are amongst the most skilled type at deluding others, while those who cannot are less successful at doing the things that psychopaths want to achieve.