Wolf
“It is a basic tenet of Judaism (and there are also scores of examples in Jewish history and literature) that ANY evildoer can repent and achieve atonement for his sins however many or degraded they are.
Except Acher, of course.”
Everyone can do Teshuvah and even Acher was able to do Teshuvah, the Bas Kol was a Nisayon for him and he was not supposed to listen to it. Had he done Teshuvah it would have been accepted. Even one who has done things that are Chayyiv Kareis can do Teshuvah and be forgiven. However we must remember that Teshuvah isnt as simple as saying “Im sorry.” We throw around the word Teshuvah not recognizing that it requires absolute sincerity and a real change in the person. Nobody can fool Hashem.