Why does Brutus decide to leave Antony unharmed and how is this decision conisistent with other aspects of Brutus' character?
Brutus is an honorable man. He does not deisre to kill unless it is absolutely necessary. He struggles with the decision to kill Caesar. He loved Caesar but he loved Rome more. Brutus feels that Caesar is becoming overly ambitious. Brutus tells his reason for not killing Antony. He does not desire to appear as bloody butchers:
Our plan will seem too bloody, Caius Cassius,
To cut off the head, and then hack off the arms and legs,
Like anger in death and jealousy afterwards,
Because Antony is but a limb of Caesar.
Let’s be sacrificers, but not butchers,
Brutus does not want to be perceived as bloody butchers. He is concerned with the image the people will have of him. He is honorable in all that he does. He tries to always to the right thing. Brutus is not envious of Caesar. He desires no gain or glory in Caesar's murder. He is simply a concerned citizen who sacrifices all that he has to rid Rome of tyranny. He is as constant as the day is long. He is not a selfish, greedy man. Even in death, Antony declares Brutus the most honorable of all the conspirators:
This was the noblest Roman of them all.
All the conspirators, except him,
Did that they did out of jealousy of great Caesar;
Only he, in a general-honest thought
And common good to all, made one of them.
His life was gentle, and the elements
So mixed in him that Nature might stand up
And say to all the world, "This was a man!"
I agree completely. Brutus is a man who always takes the "high road".
He does what is necessary but does not want to do more than what is fair and just. At times it may seem that he is reluctant to stand up for ideals and principles, but what his hesitation is an internal struggle that he must reconcile before he makes a final decision.