You know, I had somehow completely neglected Luther's incentive to join the Death Eaters until now, and consequently his stance on Muggles.
Luther, being a Halfblood, had of course grown up with a Muggle father, Arthur. His mother, Lucille, had hidden her magic from him until Luther received his acceptance letter to Hogwarts, and she had no choice but to tell him. Arthur was stunned, and soon "realized" that most of the bad things that happened to him were because of their magic (or rather, attributed everything to them instead of accepting things for what they really were) and grew increasingly resentful, and this left Luther with a poor view of Muggles. Him being sorted into Slytherin only further aided in his dislike against Muggles, as the children of Death Eaters indoctrinated Luther in the same beliefs that their fathers had taught them.
Shortly after returning home from his first year in Hogwarts, Luther was slapped for taking his wand out of his trunk by his intoxicated father, and in retaliation Luther set him on fire with Incendio, causing third-degree burns. He received a warning from the Ministry of Magic for performing underage magic, and soon after Luther and his mother moved out, and lived in Godric's Hollow for the duration of Luther's time in Hogwarts. Luther also adopted his mother's maiden name, Cunningham, in an effort to remove everything connecting him to his father. Immediately after finishing his final year at Hogwarts he became a Death Eater, and his view on Muggles remained the same until his sister's imprisonment. Not only did he realize that the Dark Lord was a monster, but he realized that just because his father was a terrible person, that didn't make all Muggles bad. Although at times he still has moments of Pureblood supremacy (old habits die hard), for the most part he has become far more tolerant and respecting of Muggleborns and Muggles alike.
I'll add all this to his bio for future reference, since it's something I should have thought of to begin with :P
EDIT: Also, Luther made an exception with his beliefs for his sister, for he cared too much about her to have negative feelings towards her. In his eyes, even though Muggles (and Muggleborns) were terrible people, there must always be good to go along with the bad, and Lilly was exactly that.