People are sometimes quick to forget the profligate use of chemical weapons, nuclear weapons and biological weapons by the likes of apparent 'White Hats' House Davion in the 1st Succession War. Comstar never did anything on that level, not even the Word of Blake fired off as many party-poppers as that.
I think it's hard to blame any one successor state for the use of WMDs in the First Succession War. The problem in the First Succession War is that every state has a large stockpile of WMDs, and once your rival starts to use them, your options are basically 1) use them back and 2) lose.
If I'm Davion, Kurita is nuking me, and I am not willing to nuke Kurita back, Kurita wins. I'm not willing to allow that, so I fire back, Kurita escalates in response, I have to escalate again, and so on. Widespread use of WMDs in the First Succession War was not due to any individual moral failures, but due to an unstable balance of power.
This is also, incidentally, why it's stupid for Clanners to get morally superior about the Succession Wars. Entire worlds were not depopulated because house lords were horrible people (though they may have been, to varying degrees), but because that's what the harsh logic of war demanded.
Has it ever been conclusively established which of the successor states first committed atrocities with large scale use of weapons of mass destruction during the First Succession War?
I don't think so, and to be honest, I don't think it matters. It would have happened independently regardless. The Inner Sphere is large enough and the fighting was brutal enough that more than one group is independently going to come to the conclusion that a nuclear strike is the best option in this situation, and once that happens, there's retaliation in kind, and the weapons are normalised.
Guess that's a matter of opinion. Kentares was bad, but WMD scar a planet for centuries. Many planets in the Inner Sphere still have contaminated zones from WMD strikes in the 1st and 2nd SWs, and some were even wiped right off the map.
Kentares was not actually that bad in terms of effects. The Kuritans depopulated a planet, sure, but lots of planets had been depopulated. Kentares was important because it was
personal. There is a massive psychological difference between pushing a button from orbit and thousands upon thousands of soldiers going door-to-door with swords and machine guns. There's also the media effect: Kentares was broadcast across the entire Inner Sphere, in all its gory detail. After a while, it's easy to be numbed to reports of planets bombed. But a video of soldiers beheading screaming women and children? That will stick.
To answer your question more appropriately, Comstar is a tension. It genuinely does beneficial work for humanity, but also acts in its own interests as an organization. This is made complex by various factions within it pushing various directions, not all of which are charitable. This tension made Comstar a fascinating actor, when not a misused plot device. The culture that built it up was supported by these tensions, creating an entity that made Inner Sphere politics quite complex and flavored. Those on the end seen less charitable were often formed by the horrors wrought by the Successor States, and sought to end them.
This is the best response, I think. ComStar is a complex organisation with its own, rather strange ideology which sometimes pushes it to humanitarian action, and sometimes to breathtaking villainy. Whether you think it's ultimately justified or not is a difficult judgement call to make.
I lean towards no, but I think understandably so. That is, I can see how there are quite a lot of genuinely altruistic, well-meaning people in ComStar, both those who aren't really committed to ComStar's grand mission but who value its practical work, and those who do believe the great mission is worth the cost. However, I think that in the long run ComStar's activities are a net negative. Even though there are plenty of ComStar operations that you can point to that, on their own, are beneficial.
