Well, that's a lot of speculation. Sure, something might have happened... but you need to look at their relationship as a whole.
Also, who is to say that your aunt isn't losing it? If she did do him in, and she isn't in her right mind anymore due to advancing age, who do you blame?
My parents are in their mid 70's. I already know that if my dad gets diagnosed with something terminal, he is going to take himself out. If my mom gets diagnosed with something terminal, they might agree to go out together. I'm OK with that because I understand their relationship.
What I am trying to say is, there is a lot more to this than what you see on the surface.