Trib's purpose is also to punish those who break laws- there are no rehabilitative, restorative or reconciliation functions in trib's justice system generally speaking. It's based entirely on harsh retribution. Death, loss of possessions.

And Trib protects people that do all the things the trib forbids in cities, but just because they don't happen to do it within four small areas trib still guarantees their safety. Hey, I just murdered this elf paladin on Eastern Road right in front of two other Maran who begin chasing me, but now I walk into galadon and this other elf paladin will protect me against the Maran that come for my head.

If thera was filled with only people of the light, trib would not really need to exist, because light people generally would not break laws anyway. Trib protects evils, and by harboring them it only encourages them and prevents their destruction. To me it sounds like they have an agenda that goes against the Maran idea of destroy evil everywhere. However, since Tribs apply their law indiscriminantly to all they are not inherently evil in so doing. They do have to be evil at times so as to apply the law evenly to all. They have to harbor a known murderer against retribution because he murdered someone outside their town and now stands next to him openly singing how he killed this elf paladin right outside.

Now you can look at Nexus.
Nexus's purpose is to protect all Thera via the laws of balance. (trib's views are no more valid than nexus's in where they originate from.) These laws have no boundaries, they apply evenly across the realm, making them far less ineffectual than trib's who have that jurisdictional handicap. So Nexus applies their law indiscriminantly to all, which also does not make them inherently evil in so doing, though at times they may commit acts of evil, they are not motivated by inherent evil within- it is evil forced upon them by necessity of duty- same as the tribs engage in, as well as Maran do. Maran know killing is wrong. They know it is evil. They do it anyway, because that is their duty. Why is it you don't condemn other Maran for the atrocities they commit against evils and forever hunt them as well?

If as a Maran you think it is okay to go after one, you should go after all, otherwise you're making rationalizations to differentiate them. Saying they raid me more often to distinguish them? I'm sorry, but there are far worse things in terms of evil than raiding a cabal.

Oh, and trib will raid other cabals too. That's a decision largely left to mortal leadership, not a set in stone policy. Pretty much all of those ideas are left to the mortal leaders, as Imms like to encourage flexibility and RP in characters instead of rigid monotony.