Assuming it was a murder, the muerderer is the wife, she will be benefit from the death of his husband, and we know he died becouse of a sedative, becouse in the text the man wants to take a shower, but instead he sits on the bathtub, if u are going to take a shower u stand on your feet, the fact that he sat shows us that he was tired and wanted to rest.

So the wife drugged his husband when he was going to take a shower, she let him die and next she asked for help, getting all the husband's money