First and foremost: Necromancers are weird.
Tevruden spent a lot of time overseeing the necromancers raising people in Naxxramas, so most of his knowledge on it is practical. “How do you raise a ____ as a death knight,” what you need (body parts, reagents, time involved etc…) His theoretical knowledge is limited to whatever the necromancers would explain when they were talking at him.
That being said, he wouldn’t use anything major, like raising someone into undeath, on anyone who wasn’t willing. But he has no issues with using it against others in combat for an advantage, or controlling undead that have already been risen.