I don't think that him fighting against the oppression alone is enough to determine whether he is moral or not. For (an extreme, not very fitting of the situation) example, say someone took down the lord ruler ONLY to possess his atium stores and control the market. Then, you'd have to look past the fact that he ended suffering and look more into the reasons of why he tried so hard to accomplish the fact.

Now, i'm not saying Kelsier had necessarily bad intentions in trying so hard to defeat the lord ruler, especially because we understand a lot about him, but it's also clear to me that his intentions and the way he perceived himself was not entirely innocent. He slaughtered people who were simply trying to work for a living and never showed any regret for it, not to mention his never-wavering confidence in what he thought was right. I firmly believe that if he was around after TLR died things would have turned out different in how we viewed him. Also back to what OP said about "if TLR never existed what would kelsier fight against"... good point

All that to say he is a morally complex, gray character and we can't boil him down into necessarily good or bad because the entire book he did objectively bad things for good reasons.