Well, it depends. A Messenger isn't a messenger when he care about saving his life given the amount of information he baits out there.
There is no reason why Bedivere shouldn't expect Martin to be either bluffing, or not the messenger at all.
To which, given that this human seems to have knowledge of vampires, there is no reason to be so dismissive of him.
He has seen them, understood their power, and attempted to bargain with them.
the "Why bother?" no longer applies as Martin is in far too deep to be brushed off as a stupid human.
Not a threat yet, but rather an annoyance to be nipped before it festers into something more.
Besides, if Martin was really smart, he'd just give the vampires what they want.
Since he hasn't, it's assumed he as some contingency plan to execute. Thus if we do not kill him, then he'd play us like chess pieces.
A fledging would kill without letting him speak. The fact that Bedivere allowed him to live this long was surprisingly noble of him.
That or he enjoys playing with his victory snack.