Call it victim-blaming if you like, but if you're foolish enough to marry a gold-digger, and foolish enough to stick your loaded gun in your mouth to demonstrate to her how easily you could die (and leave her with a fortune), then you cannot honestly be surprised when madam pulls the trigger. 

Poirot is very, very clever, but the victim here is surpassingly stupid. This is why idiots should not have access to firearms.