Like I said, at least some of what Lilith wants aligns with humanity's interests. Enemy of my enemy is my friend type deal. I am not saying Lilith is a saint, I am just saying you can at least bargain with her. We're all just trying to pick the lesser of two evils.
Besides, siding with Mephisto just means doing the same shit we've been doing for the past 3 games and we all know how that worked out. Even if we're getting slapped eventually, at least we aren't doing the same thing that we already know hasn't worked for us before.