Shishio lashed out because he was unable to get himself to come out to his parents, that's how Morio ended up with the scar. It's kind of a core part of his character, but i guess it has been a while since they brought it up. He fell in love with Morio, but was finally able move past that now.
His arc way way back revealed that the reason he lashed out as a teenager was because he couldn't bring himself to come out to his parents that he likes men.
Prince here talking about how he was sexually assaulted as a child and all the comments are just insulting him for being insensitive right after talking about his childhood trauma...
@mitsuki48 remember that the curse works by name. He seems surprised, so i think he never knew that she was the one he cursed, since he only cursed her name. Which means he was probably hired to do so.