Although it is not illegal to attack criminals and murderers, it is also not illegal for unsavory types to defend themselves against attacks. The first person to attack (even if the blow does not land) is considered the aggressor in that combat, and any attacked party can defend himself against any aggressor without fear of Karma repercussions. Thus, if you are attacked by Sheila the Murderer and she kills you, her Karma falls, and you can report your murder. However, if you attack her and she kills you, you cannot report the murder, because you were the aggressor.
Combat is considered to last until one of the opponents dies, or until about two minutes after the last blow. If you break off combat for more than two minutes, neither party is considered the aggressor until one or the other launches a new attack.