Bruce was difficult to handle, yet Barbatos still let him handle things, because Bruce was not the first Batman Barbatos had encountered. The Dark Multiverse was full of Masters.
DC Comics' dark source currents roughly possessed the following traits: they could contaminate light, but preferred flowing toward areas similarly dense with dark energy. In essence, they displayed an inert tendency toward regional concentration rather than outward-spreading contamination.
In other words, when Barbatos wanted to use Dark Force to contaminate the Light Universe, it was not like dripping dye into water. It was more like continually pushing a shot put—a heavy and laborious process.
Otherwise, given Barbatos' status, while he might not have been able to take over the entire Light Universe, with darkness already existing within it, occupying a few unimportant locations as strongholds should have been easy.
Of course, the real reason was that when Barbatos was designed as a character, his intelligence only had the distinction of being old going for it. The editors had no way to explain why such an absurdly overpowered Character Design had spent so long doing nothing and remaining obscure, only to start frantically padding his record after the Dark Knight Legion appeared.
In intelligence rankings, Barbatos did not even make the list. Forget comparing him to Brainiac—there were more than a dozen Batmen between him and Lex Luthor.
In combat rankings, transcendent beings like Barbatos, along with those in Heaven and Hell, generally did not participate. Their Character Designs were rather vague, always involving laws and the power of rules. In practice, no one had ever really seen them beat anyone in a straight fistfight. Their combat power was mostly conveyed through indirect descriptions.
The one somewhat distinctive trait was mental contamination. It sounded impressive enough, but once placed in the story,