That scene was really ambiguous. When I first saw it, I thought maybe he was crying with relief but then I realized how forlorn he actually looked. I was introduced to the notion that he knew what Tarrlok was about to do shortly after and thought it could be possible.

However, now I feel like he's crying because he failed his mission to bring equality to the world and it doesn't really matter if he knew what Tarrlok was up to. He failed and was beyond the point of caring whether he lived or died.