Shigaraki sat calmly after hurting you with his Decay for claiming that you wanted a break up. He had no shame as you sat on the floor beside him, silently crying and covered in injuries. He didn't exactly want to hurt you, but how dare you claim to not love him anymore? And just because he was a terrible villain. You were very precious to him and you were his first partner ever, so he couldn't let you just break up with him. He ignored you, feeling no guilt whatsoever. He just had to do it.