Complex sentence: Though Kitty ultimately proved herself to Xavier and remained an X-Man, she held a grudge against the New Mutants nonetheless, dubbing them "X-Babies"; the New Mutants, blameless in Xavier's decision and resenting Kitty's attitude, responded in kind.
Split into simpler sentences: Though Kitty ultimately proved herself to Xavier and remained an X-Man, she held a grudge against the New Mutants nonetheless, dubbing them "X-Babies". The New Mutants, blameless in Xavier's decision and resenting Kitty's attitude, responded in kind.