There are many (unpopular) reasons I believe this is true. Firstly, Moash is mainly hated for killing Teft, it seems that is the unforgivable thing. Yes he did the wrong thing in killing Teft, no he should not have done that. Moash killed several other people, too, such as Jezrien and Elhokar. Kelsier on the other hand, killed so many people in his lifetime, much more than Moash. Moash, as Vyre, was being very strongly manipulated by Odium (it seemed that way to me, at least), while Kelsier was being lightly prodded by Ruin, but even so most of what I hate about Kelsier are the things he did voluntarily, many of which were the same things people hate about Moash. Also, Moash would likely accept death, accept that he needs to be dead, and even if he didn’t I don’t think he would make an organization where he gives the members power and excuses as long as they follow three loose easily interpretable tenets.
While I agree with you about Ruin, I extremely disagree about the killing/murder. While some of the noblemen probably did deserve it, the vast majority of the people Kelsier killed were brainwashed by the nobles, or unfortunately happened to be Allomancers and were forced to fight people like Kelsier, who was stealing what was not his. Even the Inquisitor was an unfortunate person being controlled by Ruin (in that situation, though, it may have been unavoidable.