What is Macbeth Thinking!?!

Macbeth had a good life before he got promoted. Not only was he already the thane of Glamis, he was a general in the king's army. This probably means that he was already paid well. When Macbeth hears the prophecy of the three witches, he is overjoyed but troubled. He is troubled for the reason being, that there is already a king, and the only way for him to be king is murder. Why would he even think about murdering the man that provides for his family, is a fair and just king, and gives him a promotion of being the thane of Cawdor? This is like murdering your own father. Temptation wins again, I'm afraid, as Lady Macbeth convinces Macbeth to murder Duncan just as Eve convinced Adam to take a bite of the forbidden fruit in the Garden of Eden.