I always make an attempt to understand all that is offered to me as far as wisdom goes. But I found myself rejecting this idea that killing an Arahant guarantees rebirth in hell, whereas killing an unenlightened person does not.
Punishment requires mandate, which implies authority. That being said, if the Buddha believed taking the life of an Arahant would guarantee rebirth in hell for punishment, that with authority he has decided so.
So given that an authoritative judgement has been given on the subject, actions spark reactions. The reaction to the act of murdering someone has it's natural order, such as causing grief and guilt, as well as making it easier to murder again, the biological reaction is that a person will realize their moral crime, and find that in comparison most other moral atrocities might be excused, and they will be driven back to a primal state of acting on biological response rather than with logic, and a moral compass. And the artificial response implemented by humans, would be punishing someone for a crime, or passing judgement as to what punishment any particular crime deserves.
So if one were to say, killing an Arahant warrants greater punishment than killing an unenlightened being, this is an artificial response to the act of murder, the response is based on circumstance. To make a proper judgement you must weigh certain factors, such as value, circumstance, and reason. The issue I am having here, is the idea of making judgement over a persons life, based on value. Because I personally believe it to be impossible to place any more or less value on a persons life, compared to another. All life is precious and valuable beyond measure, and no life is of greater or lesser importance than the next.
When given the original thought, I saw only that an Arahant, and an unenlightened being were being compared in regards to punishment for their murder. No other details were given. All of this being said, with no details but their position in life being given, any judgement passed on them will be based purely on the value of their lives in comparison to eachother. Therefore placing an Arahant at higher value than an unenlightened being, leading to the decision if ever placed in a situation to choose, that one would spare an Arahant before sparing an unenlightened being because an Arahant is of greater value. This idea works only without a moral compass, which seems to be against the very idea of Buddhism.