But aren't paladins all about being a martyr?technically undead can be paladins, or at least use the holy light as a weapon and to heal, but it is very rare for two reasons:
1) it is usually used by those with a great sense of justice and/or protecting others, which is certainly not the usual forsaken personality
2) holy light causes undeads - and this includes all death knights - extreme physical pain, even if it's actually healing them. so a, say, tauren dk tank would have to endure a lot of pain each time he's beeing healed by priests or paladins. as a forsaken holy priest priest or paladin, this works double, as channeling the light would already hurt them, even if they didn't target it at themselves.