Arthas was a paladin, they control Light, Light can pwn teh darkzz mmk.
The frostmourne tainted arthas's soul, therefore not being able to resist the darkness that made him the lich king when he put on teh epic l00tz. (he shed the blood of the innocent because he was so set on saving his people + evil paower = bad)
Bolvar has not been tainted, Bolvar is paladin, Bolvar is the only person ever to resist teh darkness, as stated somewhere.
Therefore when Bolvar becomes LK, he will not be evil. Or so we hope. The frostmourne will be destroyed or hidden, or reforged, idk.
I'm not a writer for blizzard, so i don't know exactly.
So what i'm saying is, if you aren't evil or doing something in WoW that requires revenge/vengeance you will more than likely be set.
Just a theory. (Speculation is speculation)
Initiate QQ nao
/discuss