She's dead. The words were cold, Mantis pushing his own grief to the back of his mind in order to try to help Peter. --He had failed, and because of that failure, Luke had been kidnapped and Katara was dead. He wouldn't fail again. She's beyond anyone's help, now. He lifted Peter into the air, not trusting the man's own ability at the moment.
What remains is to see that those who did this pay for the rest of eternity.
Perhaps Mantis wasn't a hero. That was all right. There was a time when killing was necessary, as well, and he'd rip the ones who'd murdered Katara apart inch by inch.