Whose to blame for the massacre can be put down to, is it Tanya's fault for informing and inciting a merciless strategy for the sake of victory? (Being a beast, remember Tanya doesn't wish to commit massacre as she finds it wasteful and wants being X to stop her to prove he is god)
Or is it the Republic's fault for assuming that, in an age with no precedent for such massacres, they willingly decided to stall the empire by surrounding themselves with civilians for the sake of victory? (Thinking that beasts don't exist, remember Vianto thought there was no way for Tanya to win because massacre was never considered an option from his pov)
Now you see where the question of whose to blame doesn't have a yes or no, a right or wrong?
That's because it's conflict with differing interests and perspectives leading people to take actions which from an outside perspective seem irrational.
Both parties don't intend to cause the massacre yet both take actions that caused it, because they were thinking from different perspectives and had a conflict of interests. You can't blame the empire for not wanting their front line to collapse and you can't blame the Republic for wanting to collapse the Empire's front line. Neither are to blame for they never wished it, yet both are to blame for their actions that caused it. There is no more wrong side, they both had reasons to act as they did so you can justify anything they did from their intentions but in the end both of their actions created an unjustifiable event. Whose to blame is open to interpretation and essentially a question without answer, just which side do you BELIEVE is right.
P.S I believe In Tanya being "right" because she is the MC we follow and love when we see her interact with her superiors, her subordinates and being X and his groupies. (And annihilate her enemy, honestly she gets more screen time so she is more interesting and more fun to root for, as an outside perspective enjoying the manga for entertainment)