Technically, Arthas was evil even before he got the crown. Arthas destroyed the sentience of Ner'zhul within the crown, leaving it as simply raw overwhelming dark power. Bolvar is a different person and took up the crown for noble reasons, unlike Arthas. Bolvar is going to therefore be a different kind of Lich King.


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References:
-http://www.mmo-champion.com/threads/2287247-Why-does-the-lichking-behave-differently-now
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