@MemeSlayer09 Have you already forgotten how this manga started? The people who got summoned were a gang of murderous delinquents plus the person they were currently mugging at the time, i.e. the main character.
Brainwashed? Tricked? Fucking why? Every single 'chosen hero' was already a criminal scumbag willing to kill and torture for petty cash from the word go, that was established right at the start. What's more, their stats were absolutely outrageous, they started with crazy attribute arrays and high levels, unlike the MC, who was basically a normal human with a single broken skill that nobody really understood.
That's one of the reasons this particular manga is so much more interesting than all the other copycats in the genre. The other summoned humans aren't just assholes, or even generically evil, but explicitly the main villains of this story. They will conquer this planet and become unstoppable tyrants unless the MC mans up and abuses his skill enough that he becomes capable of stopping them. That's new, I've never seen that before, I'm pretty sure this is the only isekai out of all of them thus far that has done something like that. The human kingdom has been the villain before, that's old hat, and some or even all but one of the heroes being assholes also isn't new either. But the main character having to stop the other summoned people from destroying the world? That IS new. It's new, and interesting, and kind of exciting. We have our motivation and goal clearly established right from the start, we already know the stakes.
Why would any of them need to be brainwashed? Do you think they would give a shit about murdering innocent people as long as there's something in it for them? Odds are, the king of the human country realized he could just conquer all of the surrounding kingdoms with the three delinquents, and has promptly set out to do so, promising them money and women in exchange. That's the simplest explanation, and the one that makes the most sense.