“Murder” is the name of specific crimes in many legal jurisdictions, but it’s also an English word that is commonly used in ways that overlap significantly but not perfectly with specific legal jurisdictions.

Don't kill anyone who is not trying to murder you. If you catch someone stealing your catalyst, you use your weapon to deter a violent action of his part, not to stop the theft by murdering him. The fact that he is willing to disregard societal property norms, already flags him as potentially dangerous, s he might enforce his criminal difference of opinion by using violence. That is why security is armed; Not to shoot the sucker in the back as he is absconding with your stuff. Unless your stuff could be used to harm others, then you might be justified in using reasonable force. Always try to de-escalate, but respond in kind.