Irregular Hunter X: The Day of Sigma

Similar plot about an army of soldiers who are suddenly betrayed by their own, tyrannical leader. Both are tragedies involving genocide and destruction. However, not all hope is lost, as they have the prospect that one day, a hero will defeat this murderous ruler and avenge the fallen. 

MegaMan X and Casshern Sins are quite similar even though their stories are different. First off, both of them have a cyborg protagonist who is designed for fighting. (Fighting styles are different though) Secondly, both series feature a world in which cyborgs live alongside with humans, almost as equals. It also seems that the human population has decreased in both of them. Character desing is also something worth noting, since the main characters (X and Casshern) look very much alike.