Wouldn't call him nerfed, he says it to cover he is useless and to hide his op manipulation skills, otherwise they would be more strict in watching his every step.
[*]don't see him as a human being but as a disposable weapon
Technically, he isn't a normal person. Sort of a demigod with unreasonable powers. Not your average guy.
[*]enjoy non-human's pain
So far we didn't see that, yet.
[*]wears a mask every time she interacts with him and lies to his face while smiling to manipulate him
She's hiding the fact if or as a premeditate measure they can kill him, if he cross the line which he already is starting.
[*]plan to kill him just after the end his journey and can him powerless with the seals.
See above, he isn't a saint himself.
[*]make it so the only people he can talk to are her and his future executioner
Easier to limit his contact with people he may use to aid him, if he would manage to gain their trust or manipulate them.
[*]can lock and unlocked almost if not all his skills at will
Dunno if previous summoners had the ability, but if yes then either it failed or people that summoned told them so, reason to hide it is reasonable.
[*]can subconsciously unlock them if he manipulates her mind but the lock comes back if she rejects him in her heart.
Again, dunno if previous summoners had the same ability, but if yes then they obviously overpowered them because prejudice wouldn't exist without a cause.
Honestly, heroes summoned to other world are a ticking bomb, although largely they're good or chaotic good, something like abuse of power is presented in some stories.
For instance in Shield Hero a country of Faubrey always had op heroes summoned by them, but mostly they were shut-ins or NEET, with all sorts of sociopathic disabilities and due to either flawed genes or their fucked up point of view that place is equivalent to Slaanesh domain.
Even in comedic KonoSuba, summoned people were not exactly right in the head, one did create a giant robotic spider that annihilated anything on it's way, but due to sheer luck they died or killed themselves before making bigger mess.
After knowing all of that, in his shoes will you :
- try to honestly win their trust at the risk of failing and dying
or
- use every unethical shits are your disposal to make them break the seals to keep yourself safe ?
Yeah, trust is likely to pay out better than risking failing when trying to use unethical shits if I'm discovered, one more reason to die and them being in right to dispose a threat, which will also deepen their prejudice on next summoned "person".