Kirito and Sinon are both trying to accept and let go the terror they've experienced killing people in self-defense. The two, while they don't get along for a while, open up to each other and find support. This kind of emotional connection is at the heart of Sword Art Online. As annoying as it is to see Kirito get along with literally everyone who's codified "good guy," every arc has shown people understanding each other better through a shared online experience. That's still present in the second arc, and it's the best part of the show.
The Gun Gale Online arc falls to pieces near its conclusion as everything wraps up in the mystery. Sword Art Online is disgusting in how it treats women experiencing sexual assault: at the end of the first season, the antagonist who's imprisoned Asuna in Alfheim Online places her in chains and attempts to molest her in front of Kirito. The sequence was framed to give viewers more reasons to hate the antagonist and to root for Kirito's vengeance. In this season, Sinon wakes up in her room after finishing the competition with Kirito. One of Sinon's friends stops by to congratulate her, and it's revealed he was Death Gun the whole time. The show immediately flashbacks to a few throwaway pieces of dialogue he had as a way to pull a poor "gotcha!" on the audience. Now, the person Sinon thought was a friend announces he's going to poison her. He repeats her name over and over and starts touching her without her consent. She shuts down out of fear. This is something that actually happens to rape victims, but Sword Art Online lacks all class to cover it respectfully. Sinon's aggressor foams at the mouth like an animal with rabies. She's damseled to make us hate the person we just found out is the antagonist and to make Kirito a hero when he bursts into her house and punches the crap out of the guy. I'm not joking when I say that this episode was the worst episode of a show I have ever seen - and I've seen some shitty anime.
The following arc is a short one boasting some good animation for fights, which it needed because that's basically all this arc is about. Kirito and the gang, now including Sinon, must take down a boss and retrieve Excalibur. And since this is Sword Art Online, Kirito is obviously the only person who can have the legendary Excalibur. This arc also wouldn't be without some jokes about Sinon liking Kirito - because everyone loves Kirito.