A wrench to the back of his own head would've hurt, yes. Would he have retaliated with matched violence? Maybe. But a wrench to the back of your head? That sent him fighting tooth and nail as he mercilessly punched and beat the street thug who'd attacked you. You had a sizable goose egg but you weren't bleeding. Though this didn't seem to matter to Ichiban who slammed his fist into the already downed thug's face as you insisted and tried to convince him to stop before he killed the thug