[ He sighs, or whatever the vampire equivalent of a sigh is. Lights another cigarette. He'll run out at this rate. ]
Imagine this, then. Imagine being free of a conscience, of guilt, for a century. You don't feel bad for what you do, ever, because you have no soul and instead there's a demon in your chest, crying for blood.
Imagine that one day, someone puts that soul back in you, right next to the demon and every single bad thing you've ever done over the last century hits you all at once. You still want to kill, but that blasted soul is burning a hole right through you every moment of every day and that's what life is now. Forever.