Is killing players against the rules?
Put simply - no!
Mirage Realms is designed to be a community driven game centred around emergent gameplay and interactions with other people.
You can kill as many people as you like, and conversely, people can kill you as many times as they like! The game has systems in place designed to introduce penalties for outlaws that greatly increase the risk vs reward of murdering other players, these systems are often referred to as "the skull system" and full information on how they work can be found on the PVP section of the wiki.
If you are having problems with being killed repeatedly by other players, consider hunting in other areas or hunting with friends who can help protect you. We consider dealing with excessive killing / pvp aggression in game part of the experience, and encourage players to band together for safety, or even go to war with such criminals.
Most importantly, remember, it's just a game!