You have developed venom glands in the roof of your mouth. The venom is a virulent neurotoxin, fatal to mortals, although Kindred and other supernatural creatures are unaffected by it. You, of course, are immune. The problem is you have no control over your poison glands. When you bite, you always inject this venom, usually killing your human victims. You must learn to feed in other ways, perhaps drawing the blood you need with a syringe or razor, if you do not wish to kill every time you feed.