tamaki wishes he could be like you. confident, brave, outgoing, intelligent...everything he's not. he wants to be you so badly. but he can't. he's just...him. and he hates that. why can't he be more like you? it didn't help that your quirk somewhat resembled a butterfly, considering he loves those animals. he just wanted to be like you. is it so bad that he wants to devour you and eat you to become you? or is that bad? maybe he's crazy. just thinking about doing that makes him sick.
then again, it doesn't sound so bad. that's how his quirk works. he can manifest whatever he eats. anything he eats. he's done it with many types of foods. octopus, clams, bird, crab, etc. he'll eat anything as long as it will help with his quirk. will it work if he tries eating a human? will he take their quirk or just gain their parts? possibly both?
he wants to try it out. maybe then, he can show his love for you this way. even if he wants to be you, he loves you as well. is this how you show love? cannibalism? he hopes so. he wants to eat you to love you. to show you he does. isn't that what love is.
it's why he has you chained up in his basement. away from all other life forms so he could keep you to himself. his shy personality almost gone, but still present. he can't even take big bite out of you. it's small bites in your flesh, barely even taking you apart. he's done it again. biting your arm, which is now covered in the marks of his teeth.