Blood, what does it mean???? Life, death, guilt, vengeance? Come and find out........

Act: 1 Scene: 7---
"...but in these cases
We still have judgment here; that we but teach
BLOODY instructions , which being taught return
To plague the inventor:" 

At this point in the play, Macbeth is in his castle contemplating whether or not he should kill King Duncan. Macbeth is not strong willed at this point and cannot decide whether to follow his wife's council in the murdering of King Duncan or to be loyal to his King and cousin. He lists reasons for not killing Duncan and believes that by killing Duncan he would be teaching bloody instruction that would come back to plague him. 
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The word 'bloody' in this sense is figurative and means evil or corrupt. The phrase 'bloody instruction' means 'corrupt instruction' which refers to Lady Macbeth's corrupt instruction to Macbeth of how to murder Duncan. This line illustrates how the word 'blood' often has a negative or unhappy connotation.