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.
By being very deluded, ungrateful, overestimating oneself, e.g. wrong view, one is able, everyone not secure yet, to harm ones nurishing and most supportive "gods"... guess if you would be able. Hmmm...
'Is it true that you hold that... "Whatever a person experiences... is all caused by what was done in the past?"' Thus asked by me, they admitted, 'Yes.' Then I said to them, 'Then in that case, a person is a killer of living beings because of what was done in the past. A person is a thief... unchaste... a liar... a divisive speaker... a harsh speaker... an idle chatterer... greedy... malicious... a holder of wrong views because of what was done in the past.' When one falls back on what was done in the past as being essential, monks, there is no desire, no effort [at the thought], 'This should be done. This shouldn't be done.' When one can't pin down as a truth or reality what should & shouldn't be done, one dwells bewildered & unprotected. One cannot righteously refer to oneself as a contemplative. This was my first righteous refutation of those brahmans & contemplatives who hold to such teachings, such views.
Having approached the brahmans & contemplatives who hold that... 'Whatever a person experiences... is all caused by a supreme being's act of creation,'...
Having approached the brahmans & contemplatives who hold that... 'Whatever a person experiences... is all without cause, without condition,' I said to them: 'Is it true that you hold that...
Beings are heirs of their actions. But not only of past actions, things arises. Because then there would be no escape possible, no improve...
And yes, there is nothing wrong in saying it's a matter of ones kamma, but it's not a lineare and deterministic pattern, and not just a matter of past. For more on that one might read: Kamma and the Ending of Kamma.
The list in the question, btw., misses "strong wrong view, niyata micca ditthi*, which is of cause the very base of the others:
Abhithanani; i. matricide, ii. patricide, iii. the murder of arahants (the Consummate Ones), iv. the shedding of the Buddha's blood, v. causing schism in the Sangha, and vi. pernicious false beliefs (niyata micca ditthi).
Be carefull, all is propably still open.
[Note: This is a gift of Dhamma and not meant for commercial purpose or other low wordily gains by means of trade and exchange.]