"Why, because it was dangerous? Because you had doubts?"

Because what Istredd should have done differently is taken someone in with him. Not even Lucifer, just someone. Doom them, maybe, sure, but if he was so confident it would've been fine, then what's the problem, right?

"Scared me?" he says, his voice dangerously low. For once, strangely, this isn't so much about him. For once this isn't about him being alone. "Istredd." It's likely adding to Istredd's difficulty that Lucifer's rage becomes something near tantalizing. "Eight. Hundred. Years. The Summoned were forced into that, across the territories, together. You were alone. You went alone. You would've been alone."