Barty had very often been caught fighting in the halls. He had gotten in trouble for breaking potion bottles over some Hufflepuffs' heads in Potions his First Year. (It was a voracious acid.) - He had, suffice it to say, taken his share of beatings. He'd had bones broken. He'd actually had his skull cracked once.
His lip should not have bothered him so much. - His pain tolerance, not so surprisingly, was high.
But, it was there - showing in a very obvious way that some shit had gone down. And it had been a moment's pause that Barty knew never to take again. When you had someone tied up, you needed to move fast, regardless of your jittery nerves.
He knew that now.
And he was going to take that pleasure away from Meaghan - he'd seen the shine of triumph in her eyes - and he was going to make her hurt, so that she wasn't even capable of thinking of good things.