"From an ethical standpoint, Moe could have been the only character spared because he did not present a danger to life and limb."

I don't know about that. Remember Moe said "I tocked to Barzini. I can make the deal with him, and still keep my hotel!" Because of this statement, when Michael found out about the plan to kill him, he had reason to believe that part of "the deal" Moe referred to was that Michael would be killed. Remember that Moe (like Bugsy Siegel, the real life person he was based on) was a violent gangster in his own right: "I made my bones when you were going out with cheerleaders!" If, as was proven in Part II, Hyman Roth had the ability to kill Michael (yes, he failed, but only by an eyelash), why couldn't Moe have as well?