The only reason it doesn't happen is because you're too laid back and naive of a protagonist to just pull the trigger mentally yourself.
Well his skill isn't dependent on what he thinks, but rather what others think of him.
So, if someone even thinks of opposing MC or has a negative effect on him, they would immediately get debuffed when they get near him (even if MC doesn't think of opposing them but rather wants to be friendly with them).
But to get Buffed, they would need to be acknowledged by MC as friend.
So basically while MC has somewhat control over his Buffs, his Debuffs are not in his control at all, probably as a way to balance out his powers.
Your auto-buff auto-debuff skill is literally capable of destroying entire kingdoms by your mere presence. Of course the King is not going to just dismiss you by default just because the Hero is here. Even if the Hero is "Necessary" to defeat the Demon Lord, your presence is just as frightening as a Demon Lord, and just by passively existing.
If you ended up considering the King your enemy, as well as his entire Kingdom, what do you think would happen?
From his perspective, while the Hero may be a specialist assassin to take down a singular target. You are a walking potential nuclear bomb, not in the sense of explosive power, but in the sense of being able to suddenly propagate highly destructive and impossible to block radiation if unleashed.
So of course the King is going to give YOU the royal treatment, and not the Hero. The Hero isn't even that special, he's just a specialized weapon. He has one role, you on the other hand, you clearly are far more of an all encompassing benefit. Hell I wouldn't be surprised if all the leeway you were given wasn't because He was the Hero, but because You exist.
Well if the King is as intelligent & exceptional strategist as MC claims him to be, then the King already knows ways to counter MC.
Keep in my, MC's skill isn't spread out in a wide range, it's range is actually small, and i don't think it works on inanimate objects.
So, it's entirely possible to quietly throw MC away in a prison and let him die there.
MC can't break out of the prison because he isn't that strong, and since the effective range of his skill is small and the Debuff only work when the person view MC as a threat (or negativity affecting him with a status), then even the guards theoretically wouldn't be affected if they just stay neutral towards him like a basic prisoner.
The only reason the King would prefer to keep MC around is to,
- have someone power-up the group incharge of taking down the Demon Lord, or
- stop him from joining someone in opposition to the Kingdom