Peak Nasty for Heinlein, IMO, would be
Farnham's Freehold, in which he envisions a USA ruled by black people, and in which white people are subject to Jim Crow laws. In this black-led America, their black emperor figure eats white babies.
He pretty much lost me with that novel, sadly after reading more or less everything else, FF was one of the "well, it's almost over" reads. Helped me deal with the loss pretty good, actually.
and
The Door Into Summer, in which a grown man ends up as the caretaker of a small girl, and goes into hibernation so he can marry her when she's legal.