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he killed alot of people with his power and kind of had bad behaviour

I guess it depends on your own opinion of him in the story he was considered "bad" because he used a lot of dark and forbidden techniques during the war of his youth. He did end up killing his adopted sister and her husband but that's only because others were messing with his "reanimations" (they're like zombies he manipulates) and his personal affairs. I personally think that in the time and place and after his sect was killed he did what he had to do even if it was a bit unethical he seriously helped win the war the only reason things started going bad is because people were jealous and afraid of him, and that started to make him give into the darkness, but before all that he actually care for people and was trying to do what he seemed was best at the time.

His shijie was killed trying to protect him,so saying 'he ended up killing his shijie' isn't right.and he was the adopted one,not his shijie.

However I agree with your point,he had no other choice basically,and whatever the consequences turned out to be,Wei Ying never had any evil purpose,he just wanted to help the remaining wen sect members as he was grateful towards Wen Ning and lastly he had good morals.

“wei wuxian” and “bad” in the same sentence simply does not exist… and no im not just biased…. i-i guess.