Valgarv's hatred towards the mazoku most likely stems from the fact that he is half-ryuzoku. It's a well known fact that ryuzoku and mazoku are bitter enemies with each other and even though Valgarv is half-mazoku himself, he still has a strong ryuzoku mentality within him. The only mazoku that was worth Valgarv's time was Gaav... and Gaav wasn't even a pure mazoku. He was half human.

Gaav was the only one to come to Valgarv's aid when both the ryuzoku and mazoku wouldn't have anything to do with him. He was Valgarv's one and only friend for a very, very long time and Valgarv believed that he owed Gaav everything for saving his life and offering him a chance of revenge for his people. Since Gaav's dual-nature was being shunned by the mazoku, Valgarv's hatred for the race only grew stronger. The fact that the mazoku would have Gaav dead didn't please Valgarv one bit.

While it's never actually stated in the series, it's implied that another reason for Valgarv's hatred towards mazoku is because they never came to Gaav's aid when he was destroyed by Lina Inverse. While we know that it was actually Phibrizo who was the cause of Gaav's demise (which would be, no doubt, the major reason for Valgarv's hatred if he knew), Valgarv believed that it was Lina. So, believing that the mazoku just let Gaav be destroyed at the hands of a human didn't impress our half-breed one little bit.

What really seals the deal is when Xelloss prepositions Valgarv to join the mazoku to make them stronger (what with Gaav and Phibrizo both destroyed, the ryuzoku gained an upper hand). Valgarv is absolutely disgusted by this show of selfishness and betrayal and promptly declines Xelloss' offer. Xelloss, unsuccessful in trying to recruit Valgarv, tries to kill the half-breed. This display of behaviour disgusts the ancient ryuzoku.