Romulans aren't humans. Shinzon was the only human on Remus. If that's not a plausible reason for him having a preoccupation/obsession with humans (and Picard, who he's a clone of) I don't know what is.
That should not matter as much as the fact that the Romulans are ones who enslaved him and ruined his life. The Romulans ruined his life, not the Federation, it would have been a better motive if his plan as to destroy Romulus, not Earth.
This would be like someone finding out their wife is cheating on them and instead killing the guy who was having sex with his wife, he kills some random person instead and I am supposed to believe the cheating is the reason for this random killing, its sloppy story telling.
You want reasoning from a character who is no longer capable of thinking reasonably, rationally. That's what we've been trying to explain. Several people on this forum, with experience with mental issues/illnes (either having it themselves or knowing people that do) have tried to explain it. If you keep insisting that someone who is no longer capable of thinking straight, should still act like one that can, you've missed the entire point. His Romulan slave backstory IS the reason he's mentally unstable, one of several reasons. That's why they've bothered with it.
And honestly, there's plenty of people who consider Nemesis a good movie, albeit flawed. They're not so obsessed they need to hate it passionatly because of those flaws, they just accept them and enjoy the good bits.
Most critics and many fans do not think Nemesis is a good movie and frankly it isn't, its a bad Wrath of Khan knock off with a way worse villain.
Okay, so when he mind raped Troi, was that because he was crazy, because that seems more evil then crazy to me. It seems he did for power and a sexual thrill, rather mental illness, that scene presented him as evil, not crazy.
When he killed everyone in the Romulan Senate, was that crazy or was motivated by his desire for revenge?
The problem is the only reason people say he is crazy, is because he tries to destroy Earth for no good reason, before that, everything he did followed a similar path and set him up as wanting revenge against the Romulans.
And really saying villain is crazy is starting to come off as "get out of jail free" card for lazy and badly written villains, a villain doesn't have to be well written, because he's crazy.
I am sorry, but there are several better written villains who explore mental illness better then Shinzon ever did, he is not some well realized, in depth psyhological character, he is a badly written cliched villain, where fans and the writters have decided that "being crazy" is the only motive he needs, even though his back story gives him a valid reason to kill all the Romulans
Shinzon is supposed to be Picard's dark reflection, a Picard who went down a different path. I could see a Picard who was tortured by Romulans all his life wanting to kill them and it calls back to Picard's confrontations with the Borg, where twice he allowed desire for revenge override his morality. But a Picard who just wants to blow up Earth for no reason, no, that makes no thematic sense.
We should stop using "insanity" as a excuse for poorly written, cliched bad guys, Shinzon could have been a great villain, but he needed a better plan and motive then "I am going to blow up Earth, because I am crazy", that was really lame.