"Well, the big mistake he made, he announced himself with that silly Dark Mark over his victims" homes, and actually, the whole Mass-Murder thing really isn't a bright idea. The Reign of Terror has never been successful, either in our world, or in the Muggle one. Also, he bought into the Pureblood Supremacy line and ignored a lot of things that the Muggle world can offer. Any witch or wizard who desires power, whether in public or behind the scenes, needs to make sure he or she doesn't ignore anybody; you never can tell where a useful idea may come from."