... Mathias turned himself in defiance of God because God killed Elisabetha. Understandable, BUT he also orchestrated the death of his best friend's fiance AND he had the gall to attempt to sway Leon to his side. Dude was obviously a prick right from the start but he was suave enough that he could hide it from others.
Furthermore, the back-story ties in pretty damn well with SotN AND AoS/DoS. Elisabetha was obviously based off of Lisa from SotN, implying that either Dracula has unwavering taste or that Lisa was Elisabetha's reincarnation. This can be taken another step when you factor Mina into account. Elisabetha->Lisa->Mina. And each time one of them died (or in Mina's case, appeared to die) Mathias/Dracula/Soma went completely batshit-psycho.
With Elisabetha, he renounced his humanity and made a mortal enemy out of his best friend.
With Lisa, he started killing (impaling, Vlad the Impaler) people indiscriminately. This is supported by the time-line particularly as Vlad was born in the 1430's and Trevor and Co fought him in the 1470's. Vlad Tepes ruled mainly from... 1456 to 1462 (had to look it up) and he died in 1476. It is obvious that whoever came up with the story of SotN, and even CoD, thought about this.
With Mina, he lost control and even without the memories of his past life, he let the darkness consume him and struck out against his former friends (in the bad ending, anyway).
The character is nothing if not consistent.
Another thing I liked about the Mathias background was that it explained how the Belmonts originally became entangled in this. Mathias was Leon's friend, a brother in arms, and that makes his acts against Leon all the more despicable.
Someone like Mathias... was perfectly worthy of becoming Vlad Tepes, I think.