I have a question about God/Devil/Heaven: I have struggled to find this answer.?

I have asked Priests/Pastors and have yet to recieve an answer that actually makes sense. No one may indeed know the answer, but I am going to try to find out. Here it is: God existed before everything - he is the Alpha and Omega. With God were his angels - Gabriel/Micheal and even Lucifer (before being cast from Heaven) in Heaven. God is sinless, so I assume Heaven is sinless as well. If that is true then WHY or HOW did Lucifer sin? He was envious of God and that is why he was cast from Heaven. Not only was he cast, but 1/3 of the angels were cast out. I am soooo confused on this idea. How could Heaven, where God is, have sin? The angels, to me, seem to portray the same sinful nature as man, but how could that be in the presence of God??? Please answer honestly and openly and don't mince words....Thanks.....