Kirito's Guilt

In Sword Art Online II, Kirito comes face-to-face with his experiences from inside SAO. His feelings come crashing down on him as he realizes those he was forced to kill while trapped in the game, and he is filled with immense amounts of sorrow and guilt. 

Kirito is confronted by Death Gun and is filled with fear when he learns his true identity. Just like Kirito, Death Gun is an SAO survivor. He is a former Laughing Coffin member looking to murder once again. This reminds Kirito of the two Laughing Coffin members he killed in the invasion of their hideout, as well as Kuradeel. Even if these players tried to kill Kirito, he cannot remove the guilty feelings he carries. He cannot remember their faces, and he feels like a monster for it. They haunt his nightmares and fill him with remorse. 

While it is difficult for him to contain these feelings, Kirito eventually comes to terms with them. He explains this to Sinon later on during the Bullet of Bullets competition. He cannot change his past, but he decides to learn from those deaths and remember the severity of them. Life is precious, and lives taken should never be forgotten. That way, their importance will remain. Just as Kirito says, some things are never meant to be forgotten. This helps Sinon overcome her own traumatic past after being forced to kill a man at the post office as a child. Where she finds closure is in seeing the lives of those she saved when she had to pull the trigger. This is also how Kirito comes to terms with his decision, by cherishing the lives of those precious to him that he was able to save.

Just like Kirito has a difficult time accepting the lives that he took, he is especially hard on himself when Eugeo dies. He blames himself and ends up attacking his own fluctlight, and the power surge from the Ocean Turtle hijackers only intensifies it. He is thrown into a coma and cannot remember who he is. 

Kirito is struck so hard by Eugeo's death because this is the first time that he has faced real defeat. Cardinal foreshadowed this earlier in the season when she told him that he would not win every battle. Even if Kirito is able to defeat Administrator (the Pontifex), he cannot save Eugeo in the end. Kirito has to try and survive in a foreign land in the Underworld, and he cannot remember how he even gets there with all of his memories unblocked. He spends over 2 years with Eugeo and travels with him the entire time he is in the Underworld. For him to lose his dearest friend in that world is extremely painful and too much to handle. He needs to be forgiven by those dearest to him to recover from such a traumatic event. 

Kirito tends to blame himself for the deaths of those around him. However, he has saved many lives of those that have needed him. When a battle is a matter of life and death and carries a real-world impact, Kirito fights with all of his well-being. He powers through his suffering to protect those he loves.