Lucius was one of the Dark Lord's most trusted, valued followers, and as such he witnessed the Dark Lord's decline into instability very clearly. The Dark Lord grew more and more sadistic, more unstable and unpredictable. More vicious, and utterly indiscriminate with it – he would lash out at his followers, people who were blameless. He demanded more and more shows of utter, blind loyalty, he...he became dangerous even to those around him, and the shine of his leadership began to fade, for your father and I. But we could no longer leave. Not without signing our own death sentences, for the Dark Lord punished betrayal severely, as you know.