I've wondered if he ever did plan to actually murder Helen. I've wondered if his plan was just to slip her the sleeping pills for as long as it took to get Helen to give him control of her finances ... the day that Helen died seems to have been so chaotic that I find it hard to believe that that part was planned ...
I suspect the answers to many of our questions lie in IS's sociopathic/abusive personality
As an abuser, his singular focus is controlling and degrading his victim. It may also have been a cyclical pattern.
Because Helen is dead, we don't have a good insight into how this abuse played out. It may not have been violent, (I would not exclude that), but entirely emotional. I believe she would have been aware of it but was vulnerable.
Sadly this kind of abuse often only comes to light when the abuser badly injures/kills the victim.
Remember also that the abusers can frequently be charming to the outside world as well as to the victim.
Finally the sociopathic personality often takes extreme risks and self harms.
I suspect he enjoyed having this secret control over her. And who knows what things he did while she was unconscious.
So it may have been he kept pushing the boundaries of his abuse (drugging) and then accidentally killed her.
Sociopaths are also known to lose control. So perhaps in his acting out, he killed her somehow - in an unplanned way.
A chaotic killing is not actually that unusual for a sociopath. IS could be seen as much tidier than many.
Remember for example poor Jo Yeates who was murdered in an opportunistic fashion and then dumped.