Answer:
It wasn't necessarily the destruction of an object that originated the asteroid belt. Although this hypothesis is not completely ruled out, the hypothesis that asteroids are fragments that never managed to come together to form a true planet seems more plausible. In fact, the sum of the total mass of the asteroids, much less than the mass of our Moon, does not seem to be enough to give rise to a planet. On the other hand, we have the influence of Jupiter's gravitational field, which would tend to undo any attempt to form this planet. Another aspect is that asteroids have diverse compositions, which does not favor the hypothesis that they were once part of the same body. The asteroid belt is probably as old as the Solar System itself.