What makes an antagonist bad? Is it because they have a cool hair style and hate the protagonist (hero)? Or maybe its because they are truly evil inside.
One of my favorite antagonist comes from an anime/manga called Bleach , which is a story set in Japan where a teenage boy earns the power of a soul reaper and hunts demented souls (Hollows).
Later in the series, the main antagonist appears: Sosuke Aizen. Formerly one of the thirteen captains of the soul society who made an elaborate plan to betray them all for power. He set out, killing hundreds of souls and working around people's backs. By the time that everyone knew what he was doing, he had taken his comrades Tosen and Gin to the world of hollows. Huenco Mundo.
There, he makes a fortress and an army of advanced hollows called espada. He brings a part of this army into the real world while the rest of it distracts the main protagonist in huenco mundo. This is where it gets serious and where many strings are pulled:
Coyote Starrk (Prime Espada): He could not interact with weaker beings because they lost their souls and died when they entered his prescence. Because of this, he became lonely and split himself into two different people. He soon became enlisted by Aizen for this remarkable talent. He fought, but only out of obligation for Aizen and because of the debt he could not repay aizen for, not because he wanted to harm anyone. After his other half is silenced in battle, he loses heart and eventually gets cut out of the sky.
Barragon (Second Espada) Being the former king of the hollows, he was very "not okay" with being demoted. He only fought with aizen and worked with him to get his eventual revenge. After losing all his servants and faccion during battle, he tries to attack aizen but fades into non-existance before he can.
Halibel (Third Espada) She was saved by aizen when she was being attacked by another espada. Her personality involved sacrificing for those she loved. She fought with aizen for that sacrifice and to take care of her faccion, of whom she was very close to. After they were burned to death, she became more motivated in the fight. In the end, Aizen killed her, saying that her sacrifice would not win this fight, no matter how much she tried.
Tousen (Fallen Justice) After losing a close friend of his to the injustice of soul society, Tousen joined aizen to gain power and get revenge on the world. His goal was to destroy all those that kept of their false sense of justice, even though his form of justice was just as fake. After fighting with his former friends, he says that he is sorry for all he has done and thanks his friends for stopping him. Before he can continue, though, his body implodes, leaving a large blood stain where he had once lain.
Gin (Watchful Eyes) Gin saved a young girl named Rangiku after she had been almost killed by servants of Aizen. He then infiltrated the ranks until he was Aizen's right hand man. After years of studying him, he found a weakness in Aizen's incredible abilities. Sadly, the Hôgyoku ressurects aizen and he inflicts a fatal wound upon Gin, leaving him to bleed to death on the floor.
Aizen (Reaper of Souls) After doing this, aizen reveals his true plan: To kill 10,000 souls, collect them, and use themt o create a key that will open a door to the soul king's room. There, he will destroy soul society and make a new order. Using the Hôgyoku, which stregnthened him and gave him the powers of a hollow and shinigami, he began to slay his former captains, servants, and friends. After most have fallen, the main protoganist arrives from huenco mundo to stop him.
In doing so, others arrive to help as well. They are defeated as well, and Aizen takes gin to go and find the main protagonist home town, where many souls lie. Ichigo, the main protoganist, goes into intense training then, and goes back to stop aizen. Gin betrays Aizen and is fatally wounded by him before Ichigo can intervene. The Hôgyoku makes aizen transform constantly, but is overwhelmed by Ichigo's abilities, forcing him to the ground.
From there, he is trapped within a Koju (Seal). As the others who were still alive began to rise and tend to the wounded, Ichigo commented on, when he finally reached Aizen's level of power and had been able to sense his true emotions, had discovered that all he felt was loneliness. He speculated to another person (Urahara) that, maybe aizen had been naturally stronger than all others and had been unable to relate to anyone. Therefore he had tried to become the strongest, hoping that he would find someone to compete against.
Now, after that bit of a story (And a huge spoiler), here is my question:
What makes a good, developed antagonist?
Because, really, most "evil-do'ers" will justify their actions and never think that they themselves are evil.