Gifting a second chance.

The Dark Lord of the Slayers universe, Ruby Eye Shabranigdu, created five subordinates of great power before being sundered into seven separate pieces and sealed away. Chaos Dragon Garv was one of the five. He played a key role in the Kouma War, a battle between shinzoku and mazoku that took place 1000 years before the beginning of the series. While Garv and the mazoku were victorious, the battle ultimately resulted in Garv being sealed into a human body.

Once he was fused with humanity, Garv realised that he no longer wished for the destruction of the world that all mazoku long for. He turned his back on the rest of the mazoku and began recruiting powerful allies that would help him in his battle against the other four subordinates of Shabranigdu and their forces. Valgarv was one such ally, being resurrected as a mazoku and as Garv's retainer.

Garv's ultimate plan was to destroy the piece of Shabranigdu located in the far north and to prevent Hellmaster Phibrizo (the strongest of Shabranigdu's subordinates) from carrying out whatever plans he had for the destruction of the world. When Garv finds out that Phibrizo is using Lina Inverse to achieve his goals, he sets out to kill her before his plans come to fruition. Unfortunately, Lina proves to be a trap for Garv and when he confronts her, Phibrizo appears and destroys him. Valgarv is never present for these events but when Garv dies, his power is somehow transferred to Valgarv.

When Valgarv hears the news that his master has been killed, he's devastated. Garv gave him a chance at obtaining revenge against those who murdered his kin and gave him a new purpose in life. Garv was the only one left in the world that Valgarv trusted and that was again taken away from him. Enraged with grief, Valgarv vows that he will get revenge by killing Lina Inverse, the one he (and the rest of the world) believes is responsible for Garv's death.

This thirst for blood would drive Valgarv fully for most of Slayers TRY, consuming him with such hatred that he eventually decides he wants to see the world destroyed. For someone who demonstrated such loyalty to his master, one would think that the destruction of the world is the last thing Valgarv would seek to obtain. Garv wanted to survive and spent many years actively opposing those who would annihilate the world. Why would Valgarv forsake his master's dream rather than carry on in his name? It's entirely possible that the grief he experiences is too much for him to bear and if the world doesn't care about Garv's efforts to save it, why should he?