My guess is Sycamore was hanging onto his idealism and thought he could singlehandedly talk Lysandre out of it, or just hope Lysandre would have the smarts and the sense to stop himself from going crazy. The professor's regrets after the Team Flare plotline wrapped up felt natural, like he realized he should've tried harder or done more to stop his friend. I like the concept of a professor caught between being a naturally good guy but super close with a genocidal nutjob.

I'm more comfortable with this depiction of Sycamore than him just sitting back and doing nothing. I think what hurt their story is that it is very Tell-Don't-Show. Sycamore tells you he's upset and sad about Lysandre, but we don't see him go through the emotions and guilt. Then Lysandre drops off the face of the planet and it doesn't feel like there's any conclusion. I hope the sequel addresses this and fixes other holes too. :o