"However I think sin exists only if we believe it does."

Do you mean, "relative to a judging God"? ....or are you meaning to imply that when actions that most would consider "evil" are done, it's merely a matter of the mind -- that if we would just have a better attitude when being beaten, robbed, raped, lied to, cheated, killed, etc, we wouldn't have to accept those actions as a bad thing?