Personally speaking, I think you could hold him accountable, yes. I think there are certainly mitigating circumstances, and much of his case could be said to be tragic - but he still succumbed to the process until the point where choice wasn't his anymore. I'm not sure you could easily apply sane/insane as an attribute to him so easily, he was certainly no longer himself on merging with the Helm of Domination and transfigured, but he was quite rational and aware of his own actions. But the person he was complicit in that, as well; tragically so.