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Kaarme: He turn it off BEFORE the main battle, which makes it even more reckless to do.
His weapon also broke in the fight against the final ant.
And guess what, based on what the guild master said, the group he fought was ABNORMALLY SMALL, meaning in a 'normal' situation he'd have way more ants to deal with and likely already be dead.
He didn't know about the nature of the ant swarm, he got lucky that the group he fought was small enough he managed to win.
The guild master was not wrong to be angry with how reckless the MC was.