So Wikipedia tells me that Alexeji was the only (surviving) child of his first wife and that she was not on good terms with Peter after he left her either personally or politically, so maybe parental alienation (both Alexeji->Peter and Peter->Alexeij) is going on here?

I wouldn't be surprised if the idea that the sin of killing your own child would doom his dynasty (or get a WOMAN on the throne - Peter had changed Russian law to make that possible) was indeed one of the things holding him back.

Besides always and forever laughing at WOMAN on the throne -- and I can see how that would seem like anathema to him -- he does seem like the sort of person who would not actually go as far as killing his son, because sin (kind of like not ever committing infidelity). It does make me think of that bit in Don Carlos where Philip and the Grand Inquisitor are talking about the sin of killing one's own son and whether the GI can absolve it, though :P (Would FW have done it if he'd actually thought it would be a sin he could be absolved for? Though of course now we're talking Protestant absolution rather than Catholic, so I believe theologically you couldn't have an official of the Church do it.)

Heh, it's hard for a woman to come between the favored m/m couple when the m/m couple is actually historically canon!