The Dark Side of the Force is about passion, power, and strength; the Light Side of the Force is about tranquility, humility, and patience. None of those virtues are evil.
But why do dark side users so often kill and commit atrocities more so than light side users?
Because light side powers and feats are generally restorative and defensive; dark side powers are generally destructive and offensive. Again, neither of these are necessarily or inherently evil.
War is sometimes a necessity. We just marked the 70th anniversary of the bombing of Hiroshima and Nagasaki. Both massively destructive and offensive acts; but depending upon your point of view they could be considered either good or evil. The truth of the matter may not actually exist - as there may not be a universal truth at all.
You cannot defensively commit mass genocide - but you can allow genocide through inaction.
We saw the Jews nearly wiped out while Western powers failed to act; or the genocides in Africa and Eastern Europe. But I don't think many would consider America evil for not taking offensive action. However, going back to the points made in the Revan arc, the Jedi's refusal to act allowed the preventable deaths "hundreds of billions" of people. Is this evil? To Revan and the Republic it certainly was. But it was entirely within the light side of the Force.
For Revan, going to war, and wielding the Force as an instrument of war was not only for the greater good, but for his own individual moral good and ethical compass. He had the power to prevent innocents from dying, yet it was not his mandate and he was ordered not to do so. He used to the dark side of the Force to save countless lives by destroying the Mandolarians.
But that power, the near-absolute power he used through the Force, ultimately corrupted him; just as the light side of the Force had blinded the Jedi Council of their responsibility to their fellow sentients on numerous occasions.
So yes, Revan became "evil" from a naive point of view. But not because of the dark side of the Force, but instead, because of his inability to control himself. He became corrupted by the power he had, not the tools he used.