Adapt, Improvise and Overcome... its cliche but it's what he did and why his personality changed.
You're in a world where "friends" betrayed you, The Gods abandon you and the monsters try to kill you. Try being a soft hearted, goody good guy in his situation. you'll only end up dead like every other sad sack in this story.
If you thinking about moral high ground and such, dont... thebGod use them as tools, there is m.h o high ground. The vampire is holding on to him, helping him retain his "humanity" at least.
(No I havent read the LN/WN, So I have nothing to compare and complain about, to me, it's a good manga. Possibly couldn't have been great if it told a different story like y'all talk about, but as anew come I'm enjoying it)