Not necessarily - while Death Knights are indeed twisted by undeath and the Necromantic power that shaped them, they're still capable of feeling positive emotions and having positive traits. Thassarian, for example, still feels love for his sister Leryssa and actively rescues her in the Borean Tundra questline. Dagnar Stonebrow and Minerva Ravensorrow both feel companionship and loyalty toward the fallen Margrave Dhakar - upbraiding Baron Sliver for his aloofness. Undeath has a tendency to darken a person, but if the person in question was honorable and good in life they tend to remain so in undeath for the most part (so long as their will is their own). If someone was a scoundrel in life then undeath just makes them worse.