Many people grow up in religious homes, with religious friends, in religious communities. Atheists aren't commonly encountered at religious social gatherings, such as at Church, Bible Study, or Sunday School; which for a religious person is the main area of life where morals and ethics are discussed. This may lead to uncertainty or suspicion towards atheists, and many religious leaders view that atheists are evil, as they are viewed at the very least unschooled in the ways of God, at worst, tools of the Devil.
The influence of the religious leaders in their congregations' lives follows that many people that have never stopped to talk to an atheist about why they believe as they do, will believe as their leaders do, as that is what they are there for after all, spiritual and moral guidance.
The truth is however, that Atheism has no innate moral value, good or bad, and that while there are examples through history of atheist doing evil things, such as Joseph Stalin, the assumption that all atheists are evil is no more correct than assuming all people of African American descent play sports. Atheism is usually not something that occurs because people want to be evil, or do bad things without being held accountable, and frankly, that idea is not very well thought out.
The idea that someone's desire to not be held accountable would allow them to change a core belief is ridiculous, beliefs are something that any behavioral therapist will tell you require a substantial amount of will and effort to change, and hardly change for reasons of convenience. Someone's religious beliefs, or lack thereof, actually do not necessitate any state of moral being, as just as the example exists of Stalin being an "evil atheist", there are examples of evil being done by Christians, even large scale heinous acts of evil, such as those of Hernan Cortes, the Spanish conqueror who subjugated the Aztecs, and Adolf Hitler, who actually was a Christian, even though that is not very well known. There have been atheists who have lived quite moral lives, and done incredible acts of charity, such as well known Microsoft founder Bill Gates who is leaving the vast majority of his fortune to charity after he dies.
Atheists are as a whole defined by the lack of belief, or the skepticism of belief in theistic religions as a whole. Not all atheists scorn religion, nor do any atheists worship Satan, as that would require belief in God, which is part of a Judeo/Christian theistic religion. Not all atheists think religious people are stupid, nor do they scorn belief in religion, and some even wish they could, but simply find that they can not believe in a religion one hundred percent, that they have some form of skepticism or restraint in believing in something that they can not see, touch, or hear.
The belief that atheists are evil is mainly fueled by lack of understanding of and experience with atheists, so why not go out and see for yourself, and ask an atheist in person why they are atheist. Just try not to be condescending, and truly listen to what they have to say, and you will probably find out they're nothing like what you thought.