She could have easily taken him out then and there, swiped the chair leg from under his nose and killed him with it before taking Momoko out as well. But she didn't. Mercy was a wonderful thing, and she wasn't willing to put reputation or a kill count over a potential alliance, no matter how little she trusted him.

Her, however-

"Ariadne," she replied, nodding lightly to Momoko. "I'd say it was a pleasure to meet you, but the circumstances leave something to be desired."