That's a fair point, he was not compos mentis during the second half of his presidency. But then, suppose someone in the more advanced stages of Alzheimer's gets behind the wheel and runs over and kills a child. Are they morally culpable? I think the answer is non-obvious, and at any rate subjective.

For me personally, a necessary (but not sufficient) precondition is that the driver currently remembers what they did. This also implies that as time progresses, they lose culpability, as in time they will forget about it. It's hard to judge whether Biden satisfies this. I mean, in his worst moments (like the debate, and his recent appearance on the View) it's so obvious that he doesn't understand what's going on that I have a hard time believing he fully understands what's happening in the middle east. This, to me, would imply that he's not culpable for the genocide by virtue of simply not knowing it's happening in the first place.