Disclaimer: I don't own Harry Potter, but it might own me.
Sort of got this idea from .Fingers and his or her story, Still a Mudblood. I was also wondering about Tom Riddle and his beginnings. I had a thought, 'What if he figured that the muggles and muggleborns (and bloodtraitors, etc.) were destroying the world?' and this is what happened.
In the beginning, all he wanted to do was to save the world. It was a noble ambition. He wanted to deliver his people from their oppressors so that they might have good lives in the end. He had to have power to achieve this, of course, and although his desire for power wasn't totally selfless, there was a good reason for him to want it.
But when he was at school, the students' and teachers' mindsets would follow them for the rest of his life and despite the fact that only he really knew his reasons behind anything, everyone else thought they did and judged him by it.
He fell- slowly, but entirely- into the mold that his peers set up for him. His views were warped by the others' perception of him and he became the monster they saw.
And at the end he just didn't- couldn't- care anymore.
His power, all that power he was trying to use to save everyone, attacked the oppressors. It attacked them and swallowed them in so completely that the ones who had been the oppressors- the powerful and the strong- became the oppressed, the weak, and the pitiful.
And as he saved them and tortured and killed the ones who had tortured and killed him and his people, his dream world fractured slowly at the same time as his soul.
It was better for the people he had saved, though. He made sure that they weren't oppressed any longer. He gave them power and they used it to free themselves and take revenge on the ones to whom they had been chained. But they also used it to destroy their slavers, to rip them apart and to smile at their children whilst tearing their worlds and dreams away, because in their minds that was the only adequate punishment for having their parents ripped apart and their worlds and dreams shattered to pieces.
That wasn't how he had wanted his world to be, that wasn't his picture of salvation but he never noticed the difference between this new world and the world he had seen and wanted at the beginning because he was so, so lost and he didn't know anything anymore except for power and fear and pride.
He couldn't defend the oppressed again because he couldn't see through the haze of previous pain to notice that the oppressors he was punishing weren't anything close to oppressors anymore. It was out of control and he wasn't quite sure now what control was anyway.
For him it was a better world, though. Truly. His people were saved and had the power to save themselves (and so much more) and it was as close to perfect as it was going to get while still run by human hearts.
He saved his world in his crippled way until the next savior came along to vanquish him and his pet oppressors.
Hope you liked it!