Other mistake: Smoke rises from Karen's collar while she's lying on the table before she's injected with allium sativum. It's the shot that ends with Whistler reading her ID badge.
Other mistake: Blade talks to Karen about killing vampires using a stake, silver, sunlight or garlic and says that you aim for the head or the heart, but when he fights his mother in the temple he stabs her in the stomach with a bone and she turns to ash.
Other mistake: Right before Blade confronts Frost at the end, he is shown beating up several henchmen vampires using punches and kicks, before grabbing his sword. As he grabs his sword, you see several of the henchmen vampires "burning up" in the background. However, it was earlier established that to "burn up", vampires had to be killed by traditional means. (Staked with silver, exposed to sunlight, etc.) So it makes no sense for these henchmen to burn up from only being punched/kicked. It can't be due to his sword, as he twirls it in front of them, and only after they are already burning up.
Chosen answer: It would seem Deacon has gathered a lot of strength around himself, in followers of "young" vampires like himself. After he kills Gitano he just abducts all the vampire elders, showing his followers are a lot stronger than the elders are. So they probably couldn't have killed him even if they wanted to, not unless they want to unleash a war. They thought his pursuit of the vampire god was totally pointless, so they let him waste his time and were probably trying to find a way to get rid of him.
lionhead