A person who's murderous is cruel and violent enough to kill someone. If you are experiencing a murderous rage, it's best to stay inside and away from the public.
A murderer, someone who kills deliberately, can be described as murderous. You can also use this adjective in an exaggerated way, simply to emphasize how mad someone is, like when you describe your brother's murderous expression when he realizes you've eaten the last of his Halloween candy. Murderous comes from murder, which in turn has its roots in the Old English morðor, "unlawful killing."