Why does brutus decide to leave antony unharmed

Brutus is generally a man of honor. Killing Caesar was an emotionally excruciating decision for him. He loved Caesar but felt Rome was more important. Brutus has no appetite to kill anyone else. Brutus does not want to be seen as a murderous butcher:

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 is concerned with his status and legacy as a friend to Rome rather than a conspirator. Even in death, Antony commends Brutus’s character:

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.