You seem to have forgotten that they Shadows killed plenty of colonies and planets on their own. And rather than using Keepers, all we know for sure is that they manipulated the Psi Corps which in turn did quite a bit of manipulating of their own. And they manipulated the other races to go to war with each other. Might those leaders have had Keepers? We don't know. The Drakh are still working behind the scenes, same as they always have. But they knew that the only thing they could hold over Londo was the safety of his people.