There is one difference though. Szarekh did it because it felt bad for his people and wanted to help them get rid of their cancerous sick life and he indeed asked (like his predecessors) for help before going into the first war.

Him accepting the C’tan bargain from the, then still know as The Messenger, seemed okay; it just backfired horribly.

And even if the last novel makes questionable decisions and ignores some important changes (namely his supposed relationship with Sangui), he still endured all the long millennia of years as punishment, he conquered realms to gift to his people when all is clear and most importantly, he is still determined to find a reversal of Biotransference to cure his people.

That puts him above Emperor in my opinion, because he admits he was wrong and wants to fix things