Brutus is telling the ghost of Caesar to "be still," to rest in peace, now that Brutus, too, is going to be dead. "I killed not thee with half so good a will" means that killing himself is something he wants to do more than he ever wanted to kill Caesar.
Brutus refuses to be subject to Octavius Caesar, and so he wants to commit suicide, which he believes to be a more honorable way of dying, rather than being paraded through the streets of Rome as a conquered subject.
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