Before Macbeth returns home, what indications are there that he already has some evil purpose in mind?
The strongest indication that Macbeth has some evil purpose in mind before he returns home is to be seen in the letter he sends his wife. She reads it aloud at the beginning of Act 1, Scene 5. It tells her all about the prophecies of the weird sisters and closes with:
"This have I thought good to deliver thee, my dearest partner of greatness, that thou mightst not lose the dues of rejoicing by being ignorant of what greatness is promised thee. Lay it to thy heart, and farewell."
By "Lay it to thy heart," Macbeth means that she should keep it a deep, dark secret. Husband and wife have obviously been talking about their mutual ambitions before. From Lady Macbeth's soliloquy after reading her husband's letter, we can surmise that they have actually been talking about assassinating King Duncan but have not reached any definite conclusions or made any definite plans. This must be because Lady Macbeth is all in favor of killing Duncan but her husband is not yet committed. She says:
Thou wouldst be great,
Art not without ambition, but without
The illness that should attend it.
By "illness" she means "wickedness." She has plenty of that quality herself and is anxious for her husband to get home so that
I may pour my spirits in thine ear
There are earlier indications that Macbeth has an evil purpose in mind. When Angus and Ross meet him on the road and tell him Duncan has made him Thane of Cawdor, Macbeth is dumbfounded. His "Aside" reveals that the news coincides with his secret thoughts.
This supernatural soliciting
Cannot be ill, cannot be good. If ill,
Why hath it given me earnest of success,
Commencing in a truth? I am Thane of Cawdor.
If good, why do I yield to that suggestion
Whose horrid image doth unfix my hair
And make my seated heart knock at my ribs,
Against the use of nature? (1.3)
The "horrid image" must be the murder of King Duncan. He has not decided to do it, but he has certainly thought about it and discussed it with his wife. It was not her idea but his. Later when he tells her they will not go through with it, she reacts with fury.
What beast was't then
That made you break this enterprise to me? (1.7)
On at least one other previous occasion Macbeth has confided to his wife that he is thinking of the possibility of becoming king by getting rid of Duncan and his two sons. So Macbeth obviously has had some evil purpose in mind long before he returns home from the battlefield. They could have been discussing this matter for months.