Evil attracts kindred souls to itself; Voldemort attracts evil to itself. Evil spirits need a place to persist,1 a focal point of evil, an “axis” of evil, and Voldemort supplies this. Satan/Voldemort performs a service by isolating/concentrating evil so that our choice becomes clearer, easier. In love is Satan our brother, but those in league with evil become its slaves. Dumbledore reached out to Voldemort and Malfoy in the spirit of love, and this must be our stance with regard to evil lest we too become slaves of evil.
1When Jesus cast out the devil they needed a place to go and so he sent them into the herd of pigs.