Two episodes earlier, and after months of alpha male posturing, Rick and Shane finally come to blows. Rick manages to repeatedly punch Shane in his stupid, perpetually open mouth, and all that face punching leads to Shane staggering around with a nosebleed. We see him reflected in shards of broken glass ...
And even then he can't close his dumb mouth.
... looking suspiciously like the pale, shambling legionnaire of the undead he would soon become.
If that's too subtle for you, another twist gets revealed via Easter egg in season four. The gang follows signs promising sanctuary to anyone who can reach the community of Terminus, because characters on The Walking Dead have no sense of pattern recognition. Upon arrival our heroes are greeted by Lieutenant Tasha Yar, who offers to feed the weary travelers. In a twist that shocked literally no one, we learn that Terminus is run by cannibals and the food that was offered up was probably the parts of Ray Liotta left over after Hannibal.
"Hi, welcome to our totally normal community, where nothing bad happens to anyone!"