Commenting on the original question...
As others have said, Mathias became "evil" when God "ignored" all he had done in His service during the Crusade and let Elizabetha die. Doing it by becoming a vampire and taking control of Death was in effect the biggest "F-U" he was able to come up with.
By becoming a vampire, he declared that he would no longer be held to natural laws, such as those that took his wife from him.
However, it is arguable as to how "Evil" Mathias was at this point and for the next 2-3 hundred years. He was responsible for Leon's suffering, but he could justify the purpose of it and offer immortality to Leon as compensation for his loss. Mathias didn't become utterly evil until human's killed Alucard's mother a few centuries after he originally became a vampire. That second loss is what sent him over the deep end.
However, while I understand why the loss of his wives did what they did, his reactions are rather extreme.
He cuts off his nose to spite his face in becoming a vampire over Elizabetha's death. He loses his wife so he's going to make his best friend suffer the way he has and become a creature that preys on the innocent in order to get back at God? Pardon? Then he hides for a few hundred years so Leon's family doesn't track him down over some little grudge they've built up, before deciding to set up a small kingdom and going mad when he loses another wife.
I find Mathias' story interesting, but I'm not a fan for his reason for his original change simply because it then leaves a centuries long gap where he isn't being actively "evil" since his war is with God, not man. I'm just downloading Resurrection now, so I don't feel I can make a full comparision between Mathias and Gabriel just yet, but Gabriel seems far more sympathetic by the end of Reverie than Mathias ever is.