I think Forever is projecting onto Bad, he thinks he's going through the same thing he lived during the Happy Pills arc: losing his mind and not having control of his own body. "This isn't you" when in fact, it's the realest version of Bad we ever seen. Maybe deep down he knows that's not the case, but he's in denial because if he accepts that reality, he'll too have to accept that the Forever who exploded the whole island, that Forever filled with burning rage, that monster he wishes to never (need to) going back to, it's part of himself too.

He'll have to accept that both of them lost part of themself due to grief, and will never be the same. That's why he's so desperate, trying every damn thing, even the twisted options.