Shane is the next nearest thing to a psychopath the group has. But they live in a time where psychopaths might have an advantage (Hello? The dead WALK? That's nuts right there). He's not the opposite of Rick, he's delusional. He believes he can take Rick's family away from him and make it his own (the worrisome thing is, I sort of think he can too!).

But: That might work in a tribe of chimpanzees or mandrills, but humans have a few thousand years invested in imposing order and maintaining some sort of "society" which has certain rules called "laws". Rick is clinging - perhaps foolishly - to that idea while Shane, who never seems to think of what comes next except his own gratification (hence a ready-made family is more interesting than trying to form a bond with another one of the unattached females in the group) is less hampered by these considerations (which he thinks of as niceties that have passed on, along with a cohesive civil structure with proper law enforcement and adjudication, etc. For Shane the ideal world has arrived while for Rick, his worst nightmare has come true.