ugh im so tired of the same typical buffons that in every manga goes like "let's get rid of MC, no big deal" then get absolutely humillated. It's gotten rather repetitive actually. Wish they had at least some build up, instead of treating MC like a random ant that you just crush
Your blurring your stories together. Hitsujiya said with full confidence that they could cause the earth to be destroyed. Yuki has already shown killing intent AND they refused to share information. On top of that, their minds can't be read. The MCs may be children, but they are acting seriously and will be taken seriously.
At some point, the liability is just too high. If the MCs can cause the destruction of even a city, even accidentally, it's reasonable to want them dead. We don't know what other incidents the sorcerers have handled, but at the level of risk they are playing at, zero tolerance isn't insane. One wrong choice could get billions killed and the sorcerers have no one to fall back on.
If the MCs had acted like children and foolishly trusted the sorcerers, it would be different. Instead, they chose to play hardball, acted tough, while still being fairly weak. Now they are seen as a threat that can be contained. The MCs don't have to be evil. If the damage potential is high enough, that stops mattering.
I'd argue that after how the MCs acted, not taking action would make the sorcerers "buffons". Or at least at least hypocrites.
It doesn't help that Yuki would totally kill someone if she thought she had too.