I mean gods that are truly evil, not ones who are reflecting the social norms of the era they were worshiped in. (Zeus for example, he wasn't evil, his actions were seen as acceptable and normal at the time.)
In Zoroastrianism, the Angra Mainyu could be seen as an evil god.
Angra Mainyu...is the Avestan-language name of Zoroastrianism's hypostasis of the "destructive/evil spirit" and the main adversary in Zoroastrianism either of the Spenta Mainyu, the "holy/creative spirits/mentality", or directly of Ahura Mazda, the highest deity of Zoroastrianism. The Middle Persian equivalent is Ahriman.
I don't love Wikipedia's description, but it gets across the heart of the matter.
Also note that daevas are a whole host of malevolent gods. Interestingly, the evil daevas of Zoroastrianism battle Ahura, meanwhile the good devi in Hinduism battle the "less good" asuras. My contention, which I sadly never followed up on when I could have, is that this reflects actual hostility between India and Persia 3000+ years ago.
However, the devi in Hinduism aren't really evil, especially in early Hinduism. Some texts do treat them as totally corrupt though.