The baatzu as presented are a rope in a tug of war match between every TSR writer that's got their hands on it. They all had ideas on what they were, ideas of varying quality and compatibility that all got piled in and presented to us. Bunches of surface deep Faustian devil stories with a side of half-arsed fictional biology and lazy made up cultural info. The later is especially annoying, since the concept of talking about Baatzu art and literature sounds fun, easy to make interesting. And yet it's just another regurgitation of the party line "it is lawful and evil, lacking imagination". Oh good, like everything else.
That alignment system was poison for intelligent writing in Planescape. It puts a cap on the depth of examination, causing everything to come down to "well they're evil & lawful!". What does that even mean? What is lawful? What is evil? The baatzu are diminished by the declaration that they "want to spread evil". Why? Why do they want that? This book and all others TSR wrote about people or cultures suffer from severe shallowness, stick to their geographical work, much better.
So what should they be? Easy: Nazis. They are a nation of meritocratic ubermensch who live or die by their effectiveness and are united by a national desire to wipe out a race they see as destructive. Easy peasy. With that description you instantly know what to do with them, how they can be something other than evil just cos shutup.
- They have limited forms/ranks, but they develop new forms sometimes. They strive for racial perfection.
- They are the initiators of the Blood War, no one else. The Tanari are just a big punch bag, neither side will win. This was as intended?
- The lowest, shittiest Baatzu has been doing this for hundreds of years, progressing in a hyper competitive environment. You will not beat them at their own game. They are smarter and more competitive than you.
- They are not automatically hostile, they are not random encounter beaties. Each of them view every situation in terms of 1: How can I benefit from this? and 2: How can my race benefit from this? If the answer to both is "not at all" then they aren't interested.
- What we call betrayal, they call healthy competition. They don't screw each other over out of spite or compulsion (usually), they do it as a wonky Darwinian battle royal to weed out the weak. If a lackey were to get lucky and replace their boss they would soon get superseded by a candidate of raw competence. All baatzu are promoted because they are good at what they do. There is no such thing as an incompetent devil.
- They do not view themselves as evil. The tanari are evil and they are, if anything, performing a valuable service that you should be grateful for.
- Baatzu are sent and use expeditionary companies in the Blood War. Groups of other races that are convinced by the baatzu propaganda or have been "protected" from tanari oppression. Why not?