He’s obsessive-compulsive so he truly believes that the world isn’t big enough for all of them. He believes that she has to give up on one of them. He also believes that Claude and his curse to be is why he was abandoned by his master (this part is true). He was already abandoned as a child. Abused too. He found safe haven and to him, his safe haven left him too. He’s deeply insecure and deeply unstable. His logic won’t be logical to you unless you’ve been through the same sort of situation as him. It’s why we feel jealous despite it being “illogical”. His is just a more raw and twisted version of being jealous of a younger sibling becoming the favourite. All the mages feel it but only he feels it the worst as the eldest. He also thinks that their power is slowly killing her so he has a bit of a saviour complex going on too.
His grand plan to get his Masters attention was to off the kid she was currently taking care of. What was his thought process here, that she would give him more attention if he wasn't in the picture? Also who's to say that she wouldn't off him the second she found out what happened for hurting the child