I agree with the idea of other posters that he may well have intended for her to have some sort of accident (e.g. driving) whilst under influence of the drug or similar, which, given her dislike of driving and nervousness about it, would be more plausible than another wife dying of an epileptic fit...
RSBM
Hi Wolfclub, welcome.
I find the idea of a planned car accident very unlikely. The car was newly bought, so the dealer would become involved immediately if something technical (like cutting the brakes) was wrong.
More important, the outcome of an intended accident cannot be planned. This could differ completely from the original goal: HB might easly survive, because cars are built to survive crashes, but she might hit another car, cyclist or pedestrian and injure or even kill someone else. There would be an investigation, there would be bills to pay and claims to deal with.... and so on.