Some muggers would approach from the front, allowing the target to see them first. Very unprofessional. Some muggers would try and get clever with insults, threats. Very unprofessional. And some muggers would hesitate to kill someone. Very unprofessional.
He thought of himself as very professional. Approach from behind, knife to throat, demand payment, kill if they don't do one of a few different actions.
A target laughing was not one of the approved actions, so he dragged his knife across their throat.
The knife breaking did at least satisfy his professional curiosity, albeit at the cost of a perfect record.