>the book ends with Ender becoming the worst mass murderer in the history of the human race.
You may need to dwell some more, because there's some higher order practical and moral contention there. Guilt over things done in pursuit of survival is the victor's luxury. If you lose, and die anyway you don't have that problem. This is one of the sharp divergences that tends to exist between the soldier and the pacifist. One fights today to repent and atone tomorrow. The other risks destruction, robbing one of a future in which one may possibly avoid making the same mistakes or committing the same atrocities as one's forbearers. In this sense, the weight of being a mass murderer is shared equally by the warrior and the poet. One through action, the other through inaction. No matter the outcome, somebody's hands are covered in blood. Ender did what he was uniquely equipped to do in the moment where it was necessary for him, and those under his guidance to do it. We live not only under our own instrumentality, but under the auspices and obligations to which we are bequeathed by accident of our own existence.
Ender's Game is an exquisite portrait of how the central theme of human existence is suffering. If you live to fight another day, you suffer those fights in the future, and the scars of the past you carry with you. If you acquiesce to destruction, your suffering stops, but the world moves on regardless.
To be human is to struggle and suffer. The trick is coaxing some positivity out of the entire affair while we're around to do so. This applies at all levels of human endeavor. We all struggle against our own adversities perpetuated upon us by the "other". The biggest difference from one bit of suffering and struggle to the next, is how you define the "other".
It's weird, but Ender's Game made a lot more sense after a reading of the Bhagavad Gita. I do not know if Card set out with that intent, but the two mesh with, and complement each other in ways few texts written at different time periods have. There's also good resonance with Martin Heidigger's thoughts on the nature of man being inextricably linked with the act of becoming or Being, the characteristic quality of Dasein. You cannot change your past, you can change the future, but only in as much as the road that has lead you to where you are allows.
The Philosopher is left to wonder if it could have worked out any other way, the Faithful have their answer even if they didn't know it, and even a Free Agent, determined to sail their own way must navigate the waters pushed by the natural forces around them.