I don't think even Ilbert believes what happened in Ala Mhigo is justification for the things he's done. He just said what he did to goad Raubahn into fighting. He might consider Raubahn a traitor for "giving up" on Ala Mhigo and making it to the top of Ul'dahn society, but I think it's fairly clear he did what he did for coin, plain and simple. Envy towards Raubahn may have played a factor in it, but the coin was the most important thing.

What he intends to do with that coin is another matter. Raubahn made it to his position on the Syndicate and as Nanamo's right-hand man by earning a colossal amount of coin in the Bloodsands (gladiator pit) and buying the place. It's possible Ilberd is simply imitating his once-friendly rival, albeit using less honest means.

As for the rest... chalk it up to the Ala Mhigans being kind of belligerent in general. I forget the name of it but during the Sixth Astral Era the rest of the Eorzean Alliance (Ishgard included) had to wage a war against Ala Mhigo. They've also been occupied by Garlean forces for 15+ years.