Transfiguration requires you to hold a mental image of what you want to achieve. Surely Slughorn did not at any point have to worry about imagining himself as a sofa that retained his identity? How would he even picture this? The notion that Slughorn would have attempted to transfigure himself if there were even the slightest possiblity of screwing up and erasing himself from existence seems absurd. Furthermore, all human transfiguration would have this problem. Self transfiguration would be a near suicidal enterprise, because any practice would be potentially fatal. Here we also get the testing issue. Canon never establishes the degree to which creatures conjured from non-living matter are actually alive. We know from canon that steam breathing tortoises with spouts for tails are a possible outcome of a poorly executed transfiguration exercise. If you want to claim that the tortoise poorly transfigured from a teapot (I beleive) is fully a tortoise or alive, we're up for a very difficult conversation about what those terms mean, and where their limits are. From my point of view, it's just matter (un)convincingly pretending to be a tortoise, which I think makes a lot more sense. So death comes from the fact that, if it did work that way, nobody would try to transfigure people except with nefarious purposes, and life comes from the fact that first, there are edge cases which make the proposition fairly absurd, second, wizards generally act like life is precious, which would be a strange position for people who can create life ex nihilo, and third, a vague sense of balance with regard to the death constraint: if you can't "erase" the identity of a creature, perhaps this is just "read only space" for transfiguration purposes. The problem is that Hermione, presumably aware of this fact, would have to see no issue with indiscriminately murdering animals. This strikes me as being wildly out of character. Cedric's Labrador retriever would aslo have been an impressively callous stunt. Taking the point even further, why should this reasoning not be extended to Muggles? Can Muggles be transfigured? If not, why not? The fact that wizards in HP act like nonmagical lives are, to some degree, precious would be entirely absurd if they could transfigure a nonmagical creature into being at will. This would make more sense if you claimed owls are just magical creatures. Human transfiguration is complicated, but not so complicated that Hagrid can't manage to partially transfigure Dudley using his broken wand, or complicated enough to stop a student from accdentally transfiguring his friend into a badger. Transfiguration may well change the nature of the object, but evidently, some information about its nature is retained, since untransfiguration is a thing.