Actually it makes perfect sense, given that many Death Knights were former paladins. And since many of them were raised unwillingly and feel remorse for what they did under the command of the Lich King, individual death knights do not exactly “represents all what is death, unholy and deceases [sic]” . Furthermore given the co-operation between the Argent Crusade and the Knights of the Ebon Blade, I am sure there was a lot of time for people in the two groups to form relationships. So in summary: u mad, bro?
The first DKs were human paladins whose souls were ripped out and replaced with the souls of orc warlocks. Iirc paladins couldn’t be raised by Arthas’ Scourge as death knights but the weaker ones could still be infected by undeath. The only reason Arthas went from paladin > DK was because he moreorless lost his mind and his soul was corrupted. A corrupt paladin isn’t technically a paladin at that point. Iirc it’s really hard to raise a full-on dedicated paladin, but other classes fighting on behalf of the Light or the Argent Dawn (such as warriors or priests) can be raised easily.
It’s confusing since Blizzard retconned a bunch of stuff from WCIII. They were originally unable to contract the Plague of Undeath, however, the Ashbringer himself was turned into death knight. There’s also the fact that Arthas sent the Ebon Blade after Light’s Hope in order to raise all of the dead within it, and there were a whole lot of paladins in there.
Plus he tries to raise you after killing you in ICC, before getting interrupted by Tirion remembering his racial.