So either the Eridians are allied with that Devourer thing or something, or they were trying to prevent you from letting him out. I haven't figured that bit out.
It says in the "official" strategy guide that the Eridians sacrificed their entire civilization in order to contain The Destroyer in their little inter-dimensional "Vault". Beyond that we really don't know much. Oh, except that those Eridian things you fight near the end aren't actual Eridians, they're more like robot guards. Or something. Who really cares, they're so crappy it's no wonder their race was convinced a gigantic vagina was going to destroy the universe.
Oh and as for Angel... there are three prevailing theories about that one:
1. She's some kind of AI. This is largely based on the satellite thing at the end of the game. Also, someone said that they saw Angels face in Claptraps eye right before he was "activated" at the end. Haven't seen that one myself though. Anyways, how Angel knew what was in the vault is a mystery, so either she's some kind of Eridian virus or she's discovered something elsewhere that told her of the Vault's secret. This theory is largely supported by the fact that Angel states late in the game that the vault will not open again in OUR lifetime, and then soon after changes it to YOUR lifetime. Clearly she is something with a lifespan well beyond 200 years. Another supporting tidbit (though this may simply be trivia) is that the satellite you see at the end of the game has the designation "4N631", which when converted from l33t reads "Angel". Coincidence?
2. She's an Eridian, hijacking the satellite to watch over you. The problem with this theory is that she has/can still contact you without the aid of technology. Before you even get your HUD device, she appears to you. Oddly though, when the network is shut down she states that she cannot see you, and that "we are flying blind, friend". This mix-up of both reliance on and independence from technology really throws a monkey wrench in the works.
3. She's actually Patricia Tannis. Or more likely, Patricia Tannis after she's been possessed or otherwise infected by some trinket of Eridian technology. The main theory behind this one is that Angel seems to trust Tannis implicitly. Also, the two of them never repeat eachother's advice. If Tannis tells you to go get the third piece of the key, Angel does not, and vice versa. The last big reason behind this theory is because Angel doesn't even pitch a fit when Tannis alopes with the key and gives it to Steele. Infact I don't recall her saying anything at all. Either these two are in cahoots or Angel can somehow see the future and that was all part of "the plan".
A fourth minor theory is that Angel is some kind of godlike being trapped in the same dimension as The Destroyer, and thus she is somewhat omniscient and maneuvers you to kill The Destroyer, presumably to ease her own existance and/or allow her access to the "Vault" portal in the next opening.
Honestly though, the game is pretty weak on story.
PS: I loved the game up until I killed Flint. Then I completely lost the whole feeling of freedom, exploring, questing, and general RPG goodness. After that it just felt like playing Crysis: tearing through linear, walled in levels as fast as I could, fighting weird glowing blue alien things. But at least the final boss of Crysis was badass! Borderlands end boss was not only completely uninspired, he was dead easy to defeat. Big dissapointment. Oh well, replaying the first 3/4 of the game is still fun.