WIY, did you hear the Kotzker say it?
HaLeiVi, I think Al Ta’amin is quite relevant. It assumes that even if a person worked on himself his entire life, he might falter at the end. If so, then a sole aveira isn’t an indication that he was only sinless because of the yetzer hora’s deviousness; maybe he was a tzaddik, but nevertheless fell prey.