Evil also came about, yes. Not to dismiss it, but if a nation's legacy is exclusively the sum of blood spilled, I ask you to show me one that is not utterly drenched in evil.
And I think you're missing the point. This wasn't just some "huh, not being slaves to the king would be a neat change" theory some guy wrote down, these were intensely radical, new ideas about what it means to be a human that people took up arms over, that changed the world forever. "They did bad, too, so let's ignore the good" is a wild black and white way to view history, that utterly discounts how radical and world-shaking the ideas behind the American Revolution are. We're a quarter millenia steeped in those ideals, so we take them for granted, like that's the way the world always was.
The IMMENSE changes in human philosophy and governance the American Revolution brought about absolutely deserves celebration!