Macbeth is a murderer yes, but he is not always unworthy of our sympathies. He intially commits murder in direct opposition to his beliefs. Prior to the murder, he's consumed by guilt and indecision; he almost changes his mind, but allows himself to be influenced by Lady Macbeth. This courageous, valiant man actually proves himself to be weak of character, and these weaknesses are his downfall.
In essence, he makes a really bad butcher, and he suffers from his actions......... fevers, madness, murders planned in desperation......... he has no problem commiting these murders in respect to protecting his place on the throne, but he can't live with them afterwards. In the end, he's happy to be placed one again in the position of warrior. This type of killing is one he well understands; kill or be killed. Possibly, this is how he saw the murders he comitted, possibly this was how he attempted to justify them. None-the-less, he is a butcher, both on the battlefield, and in his quest to confirm his place on the throne. Unfortunately, I myself do pity him rather than sympathize.