It actually means "Unending Sorrow." "Krosis" can mean both "sorry" (as Paarthurnax uses it) and "sorrow," apparently. Otherwise we'd have a Dragon Priest named "Sorry." It would seem like Paarthurnax's usage is less literal than elsewhere.
The word wall for the last word of Storm Call uses it thus:
Qethsegol vahrukiv sahsunaar
Do daniik vundeheim ag
Nahlaas naal qo do
Unslaad krosis(This) stone commemorates (the) villagers
of doomed Vundeheim, burned
alive by (the) Lightning of
Unending Sorrow.
As for draugr being held against their wishes, there's some lore on a researcher who managed to gain their trust and infiltrate them. They seem more fiercely territorial than anything.