The god of evil and the goddess of good are married divine couple. The people worship them in church. But here's the thing why would majority good people worship evil god? Even though evil is discouraged and condemned by people. The evil act also punishment by the church and government. Depending on the severity of evil act is death. But they don't condemn evil god himself. But worshipping doesn't make sense.
You want the evil god to do his evil on someone else
You want to avoid war, thieves, and all the plagues the evil god loves unleashing on unsuspecting humans. You also want his wife (good god) to bless you with lots of crops, children, and riches.
The narcissistic, evil god loves being fawned over
The evil god is not going to do anything to hurt his ass-kissing followers. He's a raging narcissist and needs them to validate him. Every time he smites a non-believer and turns them into a newt, his followers laugh and tell him how funny he is, even though they are secretly terrified.
Have a Hades/Persephone type theology
While Hades had a few temples, his much more popular wife Persephone was far more worshipped if greek/roman records are to be believed. Hades kidnapped the goddess of spring-time and made her marry him. He doesn't mind being the villain of the story.
Everyone worshiping Persephone knew the story. Hades caused Fall and Winter and Persephone brought Spring and Summer. Hades doesn't really care that the humans blame him for winter. It's the way it is, and he's fine not being as popular as his wife.
Persephone isn't too thrilled with the setup either, but it's the deal struck to keep the peace between all the gods. Like most of the greek/roman stories, it's about human behavior dressed up as divine teachings. That's probably why we still tell these stories, even if no one believes in the gods anymore.
Yin and Yang:
The principles of yin and yang, at their ideal, are that all things in the universe consist of opposites in balance. Life without death is cancer. Death without life is barren. Light is warm but burns and destroys. Darkness without light is a cold void.
Mercy without vengeance is exploited. Vengeance without mercy is cruelty. Further, these two opposites are seen as needing the other to exist. So if you kill your god of evil, good ceases as well.
So people may love their goddess, but understand that the evil god must exist as well, or existence itself cannot be. Even if you could make a world of all good, another world would need to be all evil to compensate.
Perhaps a mythology like this would help explain the worship of the evil god:
The Goddess creates the world and all animals including people. But people so created are as animals, naked, eating what worms and roots they can gather. The Goddess is pleased with her pure world, free of moral dilemma.
The God, when the goddess is away from her creation, walks among the humans, and while they sleep, anoints their eyes with sacred oil, so now they see good and evil. He anoints there tongues so now they may speak He anoints their ears so they may comprehend language.
The God stays with the humans and teaches them to hunt and kill the other animals to slake there hunger. He teaches them the secret of fire to warm themselves and cook their meat. He teaches them to clothe themselves in skins and hides.
When the Goddess returned, she saw what the God had done and went to live among the humans along with the God. The humans accepted the gifts of the God and Goddess and gave thanks.
The Goddess was content again with her creation-equilibrium pleased her. But the God was discontent. So one night, while the Goddess danced beneath the moon, the God drew from his own head the flame of ambition. The God cast the flame into the cooking fires of the humans, and every human that saw those fires felt the flame of ambition alight in there heart.
When the Goddess returned she found the people waiting for her. They asked her why she had the power of creation but they did not. They asked why she could walk in the heavens but they could not. They ask why she and the God we’re Devine, but they were not. The people were no longer satisfied with their lot and now strive to grow and to gain. The Goddess saw she could no longer live among the humans so she left for heaven and called the God to follow,
The God, before he left to follow the Goddess, bestowed his last gift (curse), that of the written word, on the people so their voice might not die with their body....
Anyway, something like this mythology establishes the goddess as the creator but the evil, meddling god as the progenitor of all the characteristics that make the people who they are - intelligent, moral creatures. The goddess is worshiped as the creator and the god is worshiped as the bringer of knowledge, apprehension, and ambition. The goddess created the universe and with it humanity, but the god gave humans their nature, the ability to perceive good from evil, as well as the desire and will to act morally or immorally. So, the god is the root of all evil while at the same time the father of what separates humans and animals - the ender of innocence. In this sense he can be both worshiped and despised.
Like Alexander said, perhaps the 'evil' god still needs to be placated and satisfied, in order to avoid their wrath. Others may even see worshipers of the evil god as heroes, making sure that this god doesn't wreak havoc. You can also change 'good' and 'evil' to order and chaos. Chaos isn't necessarily evil, order isn't necessarily good. You could also change it to dark and light, or any other two similar polar opposites.