maybe a discrepancy with Blood of Youth?

I don't understand... this should be the prequel to Blood of Youth. In blood of youth they say Wang Ren Sun [Demon sect] was the best friend of Ye Dingzhi, but then he killed him. Why? If you see Dashing Youth you can clearly see that his best friend was Baili Dongjun, and he didn't kill Ye Dingzhi. So who is Wang Ren Sun? Can anyone help me clarify this matter? Which part has Wang Ren Sun here in Dashing Youth? And why did he became Ye Dingzhi best friend (and killer) in Blood of Youth?

In The Blood Of Youth, it's said that Wang Rensun was the one who betrayed Ye Dingzhi thus he took his own life. They were not best friends. It's not something to with Baili Dongjun.  In The Blood Of Youth it is said that Dongjun killed his wife, mistaking her for her sister.  In Dashing Youth, this event did not take place yet. That's why you must be confused. 

Wang Rensun is one of the 7 swordsmen who attacked Ye Dingzhi in his home compound. As he was very close with YDZ, Anshi and Wen Jun (shown briefly in later episodes), his joining of the other 6 swordsmen to kill YDZ can be called 'betrayal' in a sense.

Not sure about BLDJ as I'm only 5 ep into BoY. 

 AlessiaBoni:

I don't understand... this should be the prequel to Blood of Youth. In blood of youth they say Wang Ren Sun [Demon sect] was the best friend of Ye Dingzhi, but then he killed him. Why? If you see Dashing Youth you can clearly see that his best friend was Baili Dongjun, and he didn't kill Ye Dingzhi. So who is Wang Ren Sun? Can anyone help me clarify this matter? Which part has Wang Ren Sun here in Dashing Youth? And why did he became Ye Dingzhi best friend (and killer) in Blood of Youth?

Wang Rensun definitely wasn't his best friend, and I don't recall any mention of that being the case in BOY or DY. However, they did meet and become close friends in eps 32-33(DY). Ye Dingzhi introduces him to his family and he regularly visited them and brought them news, and was close with Wuxin(would bring him toys and play with him). And like ninetails89  mentioned, he was one of the 7 swordsmen who tried to assassinate Ye Dingzhi(Ep 39) while he was still recovering after his battle with Baili Dongjun. 

Wang Rensun was also the only one out of the 7 swordsmen who knew about his secret location. So he was likely the one who told them about it(another betrayal). There was also confusion about Ye Dingzhi's death in BOY, which was one of the plot threads that was later revealed throughout the series. All Wuxin knew was that Wang Rensun was one of the people responsible for his father's death. Although we know this was not the case now(as it was later revealed that YDZ took his own life in both series), Wang Rensun still blamed himself for playing a part in it.