Theoretically, Ryan knew that being out in the City while there were brain-eating (or whatever) zombies was a stupid idea. However, Ryan didn't always do what was a good idea. In fact, he hardly ever did what was a good idea so it really should have been of very little surprise that Ryan was leaning against the wall of a building with a shotgun in his hand and a faintly visible purple energy shield around him. For there being a zombie invasion, Ryan seemed remarkably unconcerned with being hurt.

It probably had something to do with the fact that all the zombies around him were dancing. And singing (http://www.youtube.com/watch?v=MEyWS3CTT3s).

"In hell we dance our own special a way,
let's show them how we dance while our bodies decay!"
"Do we bounce like Backstreet?"
"Not without a heartbeat!"