She'd asked him to venture with her out of the apartment, and he wasn't about to just sit on the bench and let her walk away, or pull away from her in any way. There were things happening without really happening, and Aidan was along for the ride. At least until she decided she didn't want him to be.
With the tug on his hand, Aidan was up and moving along with Evey.
And he didn't let go of her hand. If this was potentially the last contact he was going to have with her, physically speaking, he wasn't going to squander it. If she chose Preston, or if she chose nobody, he wanted to at least have the memory of her in his arms, and that kiss.