Halakha Mazmur 139:21: Chafetz Chaim, Kitzur Shulchan Arukh, Sefer HaChinukh & Shulchan Arukh, Yoreh De'ah

And this entire issur of lashon hara applies only [when spoken] against the man who is in the category of "your neighbor" [amitecha], "am she'techa," "a people who is with you" in Torah and in mitzvoth. But those people whom he knows to have "apikorsoth" [heresy] among them, it is a mitzvah to demean and to shame, both in their presence and not in their presence, in everything that he sees or hears about them. For it is written (Vayikra 25:17): "And you shall not wrong, one man, his fellow [amito]" and (Vayikra 19:16): "You shall not go talebearing among your people [be'amecha]." And they are not in this category, for they do not act as Your people. And it is written (Tehillim 139:21): "Do I not hate your haters, O L–rd? And against those who rise up against You do I strive." And one who denies the Torah and prophecy of Israel, both the written and the oral Law, is called an apikoress [heretic], even if he says all the Torah is from Heaven, except for one verse, or one kal vachomer [a fortiori argument], or one gezeirah shavah [identity deduction], or one dikduk [inference].

All those who separate themselves from the ways of the community, such as those people who have cast off the yoke of mitzvos from themselves, and do not include themselves among the Jewish people with regard to observance of the mitzvos, but are like a liberated sect of their own; for all these, including apostates, informers, and heretics, the laws of Onein are not observed, and their death is not mourned. Their brothers and other relatives dress in white and adorn themselves in white, eat, drink, and rejoice that the enemies of the Almighty have perished. Concerning such people, the Scripture says,⁴Psalms 139:21. "Behold those who hate you Adonoy, I hate" also it is said, "When the wicked perish there is joy."⁵Proverbs 11:10.

And it is practiced in every place and at all times by males and females. And one who transgresses it and fixes hatred in his heart towards any proper Israelite has violated this negative commandment. But we do not administer lashes for it, as there is no act [involved] with it. But there is no prohibition in the hatred of the evildoers, but rather [it is] a commandment to hate them, after we rebuke them about their sin many times and they do not want to return, as it is stated (Psalms 139:21), “Do I not, Lord, hate Your enemies, and argue with those that come against You.”