Both Kitty's effort to rescue him and Calvin's appearance to state the obvious were too little, too late. The Nightcrawler was, indeed, already well on his way to becoming just like the creature that had just bit him- with one major difference. Kurt was a mutant.
He stared numbly between Kitty and Calvin even as a sharp pain stabbed through his stomach. Kurt tried to lift his arm to hold his stomach but realized he couldn't seem to move it, or any other part of himself. He was still standing, but he'd seemed to have lost control of his own body. His tail twitched- he hadn't done that. He bared his teeth- he hadn't done that, either.
When he turned and hissed at Kitty he couldn't even apologize, only think over and over I'm sorry, I'm so sorry.