Should'nt there be an interrogation point?

[lestatgss]

[...]you don't believe me, do you?
http://puu.sh/vMjgT.png

[Tagamogi]

It depends. If it's so obvious that you don't believe him, he might just be stating the fact and not actually asking you a question.

I personally would probably go with an question mark here, but I've seen similar situation without a question mark, and imo, a regular period can work too and may fit the NPC's intended tone of voice better.