Hermione didn't know the implications of soul bonding, but Draco did. His pure-blood upbringing was nothing if not thorough. The ritual bound two compatible souls together for eternity. It irrevocably tied them in this life and into the next. And if the souls were incompatible - well. The reports varied, but one theme prevailed. The souls and the people ceased to exist. Painfully. It was a fair thing to panic about.