If every bad event is because of karma, how can anyone ever murder an arhat or buddha? If every bad event is because of karma, how can anyone ever murder an arhat?


  Killing an Arhat (enlightened being)


is one of the ānantarika-kamma actions.
If every bad event is because of karma, how can anyone ever murder an arhat or buddha?


Question premise might reveal an internal inconsistency - every bad event is not only caused by Kamma.

The Buddha taught that there are 5 natural laws (Niyamas), that causes physical and mental events to take place;


  1.    The seasonal laws (Utu-Niyama) related to temperature, seasons and other physical events
  
  2. The biological laws (Bija-Niyama) related to seeds and physical organic order
  
  3. The physical law (Citta-Niyama) related to the processes of consciousness, or to the nature of consciousness which recognises objects etc.
  
  4. The Kammic law (Kamma-Niyama) related to the law of Kamma, (good deeds and bad deeds and their results) 
  
  5. The Natural laws (Dhamma-Niyama) related to certain events: the general law of cause and effect, causality conditionally and unconditionally 
  Trying to further interpretate it will just carry over into  imagination. Only a fully enlightened Buddha can know the intricate workings of kamma.