Hume is making strange arguments in 3.1.1 of A Treatise of Human Nature. Consider the action of killing one's parent. Take an oak which drops its seed into the ground which "produces a sapling below it, which springing up by degrees, at last overtops and destroys the parent tree"; is that an immoral act of parricide? Hume reasons:
Is not the one tree the cause of other's existence; and the latter the cause of the destruction of the former, in the same manner as when a child murders his parent? 'Tis not sufficient to reply, that a choice or will is wanting. For in the case of parricide, a will does not give rise to any different relations, but is only the cause from which the action is derived; and consequently produces the same relations, that in the oak or elm arise from some other principles. ... Here then the same relations have different causes; but still the relations are the same: And as their discovery is not in both cases attended with a notion of immorality, it follows, that that notion does not arise from such a discovery.
Well, relations can be described with more or fewer details, can't they? We can call the situation Hume describes as
(a) "an event of some importance to something";
(b) "the death of a biological organism";
(c) "the killing by an offspring organism of its parent"; or finally
(d) "a human child killing his father."
The first three descriptions are not precise enough to be "attended with a notion of immorality," but (d) is. (d) has enough information to let us evaluate it ethically.
Hume goes on that if immorality were something perceived by the intellect, then it would first have to exist "out there." But then a wolf killing a farmer's sheep would still commit a crime, even if the wolf is too stupid to realize it. A human sheep thief is as guilty as a canine one, except the human knows it, and the wolf does not. And that doesn't make sense.
Again the matter comes down to the description of the event. "The killing of a sheep by a living creature" is not sufficient to tell us whether the killing was criminal or not. "The killing of a sheep by a human thief / wolf" is. Why? we may ask.
We can establish a simple correspondence: there is a crime if and only if punishment for any of our 4 reasons is warranted.
Clearly, a wolf cannot be rehabilitated, unless the punishment is part of the process of taming it. (Even then, you would teach the wolf to fear the master not love him, as rehabilitating humans does.) It is ordained from above that a wolf shall find sustenance by eating sheep, and there is nothing anyone can do to change that fact of nature.
Retribution to the wolf, as if dispensing "justice" to it for having dared to raise its paws against its superior (or something like that), is blasphemy.
Other wolves will not come to fear punishment by watching you punish the guilty wolf; so they cannot be deterred.
Nor, finally, can wolves be meaningfully condemned, this being reserved for human beings who are part of the moral community. One cannot kill a man but only if he turns into a man dog or wolf, as it were; and then only lawfully by order of a judge, etc. But one can kill a wild wolf for any reason at all; or indeed for no reason.
Thus, since it can be useful to punish humans but not wolves, we conclude that humans can be morally guilty, while wolves cannot be, and this by reason alone.