The prohibition of lashon hara only applies to someone who is considered “part of your nation” which excludes an apikores. An apikores is someone who denies the entire Torah came from Hashem or even doesn’t believe that one mitzvah or one drasha, like a kal vachomer or gezeila shava comes from Hashem. Now the avos diRebbe Nosson (16:5) says “A person can’t say I only love rabbis but simpletons (am haaratzim) I don’t love. You have to love every Jew besides an apikores and people who convince others to sin, as King David said, ‘Those that Hashem hates I hate’ (Tehillim 139:21).” An apikores is hated by Hashem because he willfully acknowledges Hashem’s Torah and denies it so it is permissible to speak out against him and degrade him to his face and behind his back since the verse says these prohibitions only apply to those who are acting within “your nation” but a non-observant Jew who doesn’t know any better, he’s a simpleton, who doesn’t know or understand the severity of Torah life should be loved by every Jew just like Hashem loves them and one cannot speak lashon hara about them.
One must have heard himself, blasphemous talk coming out of the mouth of the would be apikores to be allowed to speak out against him or if it is known everywhere, at least throughout the city, that a certain person or group is an apikores then it’s permissible to speak out against them, for example Jews for Jesus or, an extreme example, Hitler and Nazis who everyone agrees they were bad. But if you heard secondhand information that someone might be an apikores then you are allowed to take precautions and even tell others to stay easy from him for them until it’s confirmed but you can’t just speak out in public against them until verified.