What I think you're asking is: Why didn't anybody realize she could kill Endbringers?" He, because it had taken Dragon less than a second to identify the man regardless of their precautions, gave a half sigh, half laugh before continuing.
"If you ask the question out loud like that it pretty much answers itself, but I'll take a shot at it anyway. Three things you need to keep in mind about Twilight Sparkle. First, she's cute. I mean she's absolutely F-ing adorable." A graphic came up on the screen next to the agents head showing a picture of Twilight reading a book at a desk with a somewhat miffed expression that was, Dragon had to admit, dangerously cute.
"Yeah, just look at that, she's like the ideal cuddle toy every doll maker has been trying to make since forever, blown up just big enough that you don't have to bend down to pat her on the head, and brought to life. Then she starts talking and she's this friendly, geeky, cheerful, awkward young lady who's a little weird, but just enough to make her charming. The whole package is completely disarming and even when you know what she can do, it's way too easy to forget when your inner child just wants to pet her and feed her carrots and your parental instincts want to give her a hug."