In the world of Angel quickly became a fan-favorite character. He was so popular that he was actually given his own series away from Buffy and the rest of the Scooby Gang.
While Angel comes across as an angst-ridden love interest for Buffy at first, fans slowly began to realize that there was a lot more to him that they first thought, and some of it wasn't good. As the character developed over Buffy the Vampire Slayer and Angel, he got increasingly worse in many ways, some much less forgivable than others.
Demonic Past
At first, Angel just seemed like a hot, heroic vampire who wanted to be better and was obviously attracted to Buffy. The problem was his past. As fans learned about Angelus, the vampire name he used before becoming good, the more fans realized just how awful a being he truly was.
Angelus killed innumerable people over the course of his existence, torturing and tormenting as many of them as possible just because he could. This history definitely impacted how people saw him.
Evil Bubbling Under The Surface
How Angelus became Angel was also something of a problem in how he was perceived. After an incident with the Clan Kalderash in 1898, Angelus had his soul restored to him, meaning he now felt real guilt for everything he ever did.
But having a soul didn't mean that everything that made him evil was gone. Angel was still capable of great violence and destruction, something that was demonstrated when his soul was suddenly gone again.
Kept Losing His Soul
On more than one occasion, Angel lost his soul, which immediately caused him to revert back to being Angelus. There seemed to be more than one way to make this happen, indicating just how precarious him being on the side of the light truly was.
To make matters worse, the Kalderash curse that gave him his soul back in the first place could be broken by experiencing a moment of pure happiness. In his case, that ended up being spending the night with Buffy.
Old Man, High School Girl
This was a watershed moment for a lot of people, and not because the heroic vampire turned back to evil because of it. Buffy was in high school when Angel took a shine to her. As a vampire, he was approximately 260 years old when they met.
If you want to go with his human age, the guy was 26 when he was turned to a vampire. Any way you cut it, 26 is a little old to be dating high school girls, let alone sleeping with one and losing your soul at the age of 260. It was more than a little creepy.
Left For Los Angeles
After everything they went through, Angel left Buffy in the end to seek his destiny in the city of Los Angeles, which wasn't trite at all. Angel in the City of Angels? Does Cleveland not need defending from the forces of evil?
Regardless of where he ended up, the reality was that Angel put Buffy through hell, then took off. Even worse, he couldn't leave her alone and kept bugging her to see if their relationship was still remotely possible.
Sacrificing His Enemies
Once in LA, Angel ran afoul of a multidimensional evil law firm named Wolfram & Hart. They caused Angel and his allies never-ending problems and were behind more evil plots than even the Scoobies knew about. At one point, Angel had enough and went rogue on the situation.
One moment, in particular, saw him locking a bunch of Wolfram & Hart executives in the boardroom with Drusilla and a resurrected Darla who then did what vampires do best and killed them all. Angel couldn't even blame that one on a missing soul as Wolfram & Hart brought the worst out in his soulful self.
Spike
Since Angelus sired Drusilla and Drusilla sired Spike, Angel was, in a sick way, Spike's grandfather. One of the few constants between Angelus and Angel was their seemingly mutual dislike of Spike, and their desire to torment him on some level.
Even after Spike regained his own soul and became a hero, too, Angel still treated him extremely poorly. That was unfortunate as Spike turned out to be one of his more reliable allies. Well, in his own way.
Took Charge Of Wolfram & Hart
Eventually, Wolfram & Hart took more of an "if you can't beat 'em, 'em" approach to dealing with Angel by giving him their office in Los Angeles. Angel's intention was to take the machine apart from the inside but that changed as he was drawn deeper into it.
He lost his course, straying from the righteous path and becoming mired in Wolfram & Hart business. The only thing that made Angel wake up to that reality was Cordelia coming out of her coma and telling him to smarten up.
Damned Los Angeles
The end result of his time in LA was a war that essentially damned the entire city of Los Angeles. Not just the people in it but the entire physical infrastructure of the city. Angel ended with what was left of his team taking on the forces the Senior Partners of Wolfram & Hart had mustered against them.
The rest of the story played out in the comics, and eventually, things worked out. Angel probably still should have found a way to fix things that didn't require a trip to Hell for one whole city.
Let A Lot Of Friends Die
By the time Angel's run on TV had come to an end, he had let a surprising number of his friends and allies die. Some of them were preventable and many of them were unforgivable.
This unfortunate slaughter started when he was Angelus and killed his own human family to kick off his new undead life. It was a pattern that followed him in one form or another through his time as a vampire.