Originally published 3/27/11, 10:54 am.
Sefer Mitzvat Moshe - Kitzur Sefer Hareidim 2:6
It is forbidden to hate anyone in Israel sheneemar "lo tisna et achicha blivavecha...."
However if you see someone doing a sin and that person refuses to accept hochachah it is a Mitzvah to hate him. As it is written, "halo m'sanecha Hashem esna"
I would personally modify this mitzvah to "hate" to something more akin to "disapproval" in the spirit of Loving the Sinner but hating the Sin.
Just note that, at times, being too forgiving may devolve into enabling. In other words, there is no "Chessed" IMHO to give liquor to an alcoholic no matter how much the alcoholic may plead for it. Similarly, supporting a Sinner can foster his or her self-destruction.