I mean, it's not like magic that alters people's minds exists in this world or anything.
Oh wait...
He TRIED a nonviolence by imprisoning him first. Subs stays locked up till Amelia finishes. Nobody gets hurt. But then he broke out, his non-violent solution failed. Now it's time to violence.
I'm not saying he's a good guy. He's a Bakugo-ass character. He's violent and rash. He is literally a caged animal. And his actions are in keeping with that kind of character.
They also don't know what did do it. He's not supposed to be able to take it.
You're acting like the test is some kind of benevolent spirit, and that they should be treating him being able to take it as confirmation that he's a good and worthy person. But it's not, it's a magic trap thing that he is somehow able to circumvent the rules of. That's suspicious, not reassuring.
I feel like you already stated the solution.
He took the first trial, and came out smelling way more evil. Therefor, letting Subaru take the trials must be evil.