It isn't about Lara or Thomas trusting Harry, they can.. It's about Harry trusting them. It isn't a matter of morality either, back to the pet tiger analogy, as much as you love it and believe you can trust it, if it is hungry enough, it's natural instinct wins, and it could eat you. Vampires do what vampires do, that is what Eb is telling Harry. Now whether this is from Eb's own personal trauma, as in one killing his wife, or from the general experience and knowledge of his 200 plus years we haven't been told yet.
Yes. I like the 'pet tiger' analogy, it really fits in many ways. Every now and then you hear about a tragedy where somebody who thought they had tamed a wild predator finds out the hard way that it's still what it is. As the late Rush Limbaugh was fond of saying, "A snake is
always a snake, a tiger is
always a tiger." Even when the wild animal is safe enough around its owner, it can be dangerous to others, such as that case a few years ago where a pet chimp tore a visitor's face apart. Plus those incidents where a pet python gets out of its container...
Now since Thomas is also a human being, his sapient mind can restrain the animalistic predator...as long as it doesn't get too hungry. But once it crosses that line, it can override the human and go hunting, and any human available is prey.
En and Bob have both warned Harry about this, but it's not a message he wants to hear.