I think most Christians distrust atheists because they think atheists deny God and therefore having no moral rock; or going even further, that atheists deny God because they want to be free to be wicked.
My view of Atheists is that they want to follow their own will. This is the fundamental of the church of Satan. (Do what thy wilt shall be the whole of the law). Anton Levey of the Church of Satan is quoted as saying "The purest form of Atheism, is Satanism".
It boils down to
"Do what you want"
"Do what God wants" (THY will be done)
This doesn't mean that an atheist has to be so called "wicked".
Atheists often hold morals, charity etc., which all seems great on the foreground. I don't see atheists as all wicked and evil in their works (as a general), but I see them as casting themselves as their own "god" of sorts. aka "I'm in charge of me, my own will, etc." rather than giving up their life to do God's will. This is what "The angel of light" did in Isaiah 14. He "Cast himself above God, and above the heavens".
Christians do these things to glorify God in his will. If an atheist helps an orphan, it's (more or less) probably because the atheist feels bad or wants to. If a Christian helps an orphan, they do it to glorify God, and follow his (God's)will first, and also because the Christian feels bad and wants to as well.
A Christian (is supposed to) live their life in God's will both in spirit and works. (Thy will be done)
An Atheist lives their life according to the way they want. (Do what thy wilt)
Hopefully this makes sense, LOL.