So - it seems the wedding was still on - even though the relatives had not been advised of an engagement !!
so my thoughts, that the catalyst for the killing was that Helen was going to leave him, were incorrect.
Was his illness the trigger then ?
He gets a scare ( cancer was it ? ) and suddenly starts to think that time might be in short supply for him.
OK, they are getting married, after which time he can work on getting Helen to let him have more access to her money - over and above the £600 allowance he is currently getting. But this could take months.
So instead he begins the drugging.
Then, when he gets the all clear, he is so far gone with his plan, that he thinks there is no need to go through with the marriage and the slow task of getting access to the money.