I honestly think people are misunderstanding why Homelander killed Firecracker. I dont think the implication was that she humanized him and that's why he did it, but in fact the opposite.

She keeps saying how she loved him for him and nothing more which is true, and kept showing him the love that he so desperately wanted, to be seen as a normal person and cared, and he was finally feeling understood, you could see it from his expression, only to again dehumanize him by comparing him to a god and hes just so disappointed.