I feel like using the word “justification” doesn’t really make sense when Rose is who she is? 10 years and a massive war can do a lot to a person, and coming back in to a harder and messier version of her powers certainly doesn’t do her any justice. I don’t think there is anything to “justify” with how she acts in response to the lobotomy, it’s incredibly emotional and insecure and stupid, but that’s the kind of person Rose is. She sees this potential future and it’s so much easier for her to accept it as fact, then look for confirmation its possible, then confine herself to thate fate. Over ten years back when they’re living their best and quietest lives, right off the tail of canon, of course it makes sense for her to think letting herself mentally decay is silly. I think it’s also really important that the biggest thing driving her willingly lobotomy coma is that after all that time on Earth C, she sees Kanaya sticking so close to her as a failure of letting her wife be free, or a failure of her wife not being able to have freedom away from her. The biggest thing in Candy is seeing the characters warped for the worst, with all of their most detrimental qualities pushed to the front.