Though Edward knew there was some humor to be garnered out of her promise, he was completely serious when he responded, "I'm not promising anything." Not that it was necessarily his fault; Edward was not a soldier when he or his kin were in danger, though at one point in his life it was the one thing he'd wanted to be. He was all guts, all instinct when on the hunt, and enough of his control would be focused on not harming either of his companions and protecting both as well as he could, though Bella would always be the priority. Even if he did agree to promise he wouldn't "go crazy", it was mostly likely he would break it.

"As for that thing in the mirrors," he said, the word thing clearly gripped by a hatred that was evident in the way his grip tightened on hers, "it may be an acceptable idea. It's a better plan than wandering the hallways and hoping to come across something none of us have any experience with."

[To here.]