Both Angeal and Genesis were part of the original Jenova Project that eventually birthed Sephiroth as we know him. Genesis was the 1st, but Hollander was in charge of the program at the time and was careless. He pushed Genesis's body too far and it started to degrade, because it couldn't handle what Hollander had done. I get the feeling Genesis also felt kind of inferior to his friends and always felt like he had to prove something (recall the fight with Sephiroth in the training room). I assume he also went a little crazy as his body degraded, and he went nuts with the power he was developing, even as his body was dying. He originally left to find Hollander, then came back to try and get Hojo (hence the attack on Shinra).
Angeal received similar treatment but he was stabilized, so he wouldn't degrade. That's why Genesis asked Angeal for his help to prevent him from dying. When Angeal went to Wutai, I think he knew what he might find (Genesis copies, and/or Genesis himself) and what he might have to do. Shinra tried to make it look like Angeal had betrayed the company and sided with Genesis, but I think he was trying to bring him back and save him. Clearly, that offer was rejected, as they end up fighting later after Angeal returns. That, or he just realized Genesis had gone batshit and needed to be stopped.
Genesis thought Hollander could save him, but I think Hollander never cared one way or the other, he just wanted to keep up his experiments, to the point that he even did it to himself, and he knew Genesis would not only not kill him, but would protect him, unlike the rest of Shinra who wanted him captured.
The Genesis copies make more sense when you recall all the Sephiroth clones in the original FF7. Yes, most of them had tattoos and were part of Hojo's experiment, but he knew that the Jenova cells and other tests would have that effect on them, so he was kind of making Sephiroth copies only in a controlled lab setting, and after Sephiroth had supposedly died.
What's interesting is that the only time in the original FF7 that you see the real Sephiroth is in the Northern Crater at the final fight. There was a book released in Japan detailing the story of the game, and I read a translation of it.
Remember how Sephiroth never appears until after Jenova breaks out of it's holding container in the Shinra building? If you recall in the snow village where Gast's lab is, the records say that Jenova could assume any form and tricked the ancients into thinking it was one of them, but it was really infecting them and killing them off. They sealed it in the crater to prevent it from destroying the planet.
According to the official Japanese compendium, Sephiroth's being thrown into the lifestream must have pushed his body over the limit, too, like Genesis (recall his one black wing), only he had Jenova with him when he went in. Between that and the cells in his body, he was more Jenova than Sephiroth, and he was manipulating the rest of Jenova in Shinra (and later over the world) to take his appearance and kind of go on a killing spree. This is why "he" is always dropping parts of Jenova for Cloud and the gang to fight - because it was never Sephiroth, just Jenova assuming his image.