I don't think I missed anything. After the first time he slept with Buffy he should have realized what a danger he was and ended it all. The Powers that Be (or whatever they were called) could have just called on another hero to do battle for mankind. Specifically a hero who wasn't prone to go to the dark side. I know he wasn't always Angelus and was undercover at times but he did a lot of damage along the way as a hero.
Not accurate. They couldn't just pick another champion as Angel was at the time (later Spike had one as well) the only vampire with a soul to fulfill the Shanshu Prophecy. It was his ultimate goal for redemption and his way of making up for the pain he caused as Angelus. He felt obligated to save the world. To off himself would not have even made sense one bit and the PTB couldn't simply change prophecy. Also, it was unclear if Spike could ever had fulfilled the prophecy at all.
The show simply didn't just decide to make him go dark. Wolfram & Hart believed that Angel was the one to bring on the apocalypse versus save the world from it because the prophecy was not clear. They had a hand in Angelus's return on more than one occasion. It's not like the show was trying to turn him evil as you put it.
Angelus was a very small part of all he did and if you look at the central themes, being a hero was not always black and white. Several character through both shows made questionable decisions as "heroes". In fact, the entire story line behind Wolfram and Hart made that abundantly clear there was a fine line between good and bad. Even on Buffy, you later discover her powers came from a demons and the Watchers Council made questionably moral decisions.
You see the opposite is true. Only a coward would have taken the easy way out and Angel chose to do the hard thing and take on the responsibility to save the world from the apocalypse foretold in the prophecy and deal with his own inner demons. That's what a real hero would have done.