Board Thread:Skyrim/@comment-110.168.240.168-20140424104245/@comment-206.80.199.220-20191031180758

Paarthurnax is the one that tought mankind the Thu'um. Killing him technically means killing the one who granted you your power, that is ethically wrong because he also granted you to kill him and every other dragon permanently. Using something he gave you against him is not good and potentially a act of evil.