It was a group of adventures that saw a plant girl monster that lulls them to sleep and kills them, but the rest of the party won't let the main guy kill it. He then goes back behind their backs and it turns out she is a delinquent, not a nice girl, and so he kills her and gets beat by his party for it.
Similar scene in Konosuba, not sure if it's the same one you're thinking of.
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