Lucifer never lied. That was what humans got wrong most often, where they kept making mistakes. They thought that if they didn't listen to the lies, they'd be alright.
But Lucifer never lied.
He didn't have to. He didn't even twist the facts, or avoid the realities of a situation. Gabriel had asked him once after the fall, what he thought of humans, of their propensity for evil.
His brother had smiled through the bars.
Tell them the absolute truth. And they will find their own way to Hell.