The Angels are sort of explained in the endless amounts of superfluous material.
Essentially, an ancient creator race flew around depositing white and black eggs. From the white eggs, emerged the angels, creatures with crazy appearances and crazy powers. From a religious point of view, consider these creatures as ones who ate the fruit of the Tree of Life.
Humanity comes out of black eggs and does human type things. Supposedly, they have the fruit of the Tree of Knowledge.
There weren't supposed to be two eggs on the same planet because it causes the angels and humans to instinctively fight against one another. Earth was a white egg planet, and there was another planet that had a black egg. The two planets collided, leaving behind a fraction of the black egg while the rest of it became the moon.
Thus, Angels hate humans and humans hate Angels. Can't help it. They want to be the sole race on the planet. The angels think that the humans have Adam. So, they continually try to rescue their leader in order to wipe them all out. Perhaps it would cause humans to be goo. Who knows.
Instrumentality was a Gendo/SEELE concocted plans. The angels really aren't a part of it at all.
Yeah, when I found this out, Evangelion actually made sense. Le gasp.