Hellboy is questioning Trevor's assumption that because a creature was born of evil, they must themselves be irredeemably evil. The horn shaving, as discussed elsewhere, represents Hellboy's rejection of his destiny as a harbinger of doom; as such, what Hellboy is suggesting is that Ruiz might not have been beyond redemption. He is also pointing out that he, himself, as a literal demon summoned from Hell, might well have been assumed to be evil on the face of it, when this is clearly not the case. As such, he sort of is poking fun at what he sees as Trevor's hypocrisy. Perhaps there is some guilt involved here, too, considering that he did end up killing Ruiz rather than finding some way of avoiding it.
This also sets the stage for Hellboy's temptation to join Nimue later in the film. She claims that she will allow creatures like Ruiz and Hellboy the chance to be themselves, without being simply dismissed as evil and beyond redemption. Of course, this would come at the expense of having to accept her authority, and her philosophy is not about giving monsters a chance to reform so much as not caring what they do to the world as a whole, but setting the stage for that plot is the purpose of this whole movie introduction.
As a side note, Hellboy seems to be mostly right. While a good number of specific monsters are evil and destructive, and the legions of Hell seem hostile as a general rule, it's worth noting most of that the evil creatures we see are driven by very familiar emotions, rather than simply wanting to wreak havoc. Thus, Nimue is evil, but one of the other witches can be an ally. Indeed, we later see a very similar case to that of Ruiz, albeit with a wereleopard instead of vampirism, which at least heavily implies that Hellboy wasn't so far off the mark. If a good number of monsters shown are violent and destructive - well, so are a lot of the human characters.