This episode seems like it was about Sinon, but it was about Kirito, and Kirito’s past. I’ve said this before the season aired, but one of the things this season does, which is not all that subtle, but it’s also not entirely spelled out, is using Sinon’s character, and her character development, to actually explore Kirito, and to retroactively explain his actions and fill him with more depth.
When Kirito is talking to Sinon, he’s actually talking to his past self. “I’m fine with going there and dying on my own. I’d rather die than be weak.” You can also replace that wit “I’d rather die than be depressed”, but who does that remind you of? Why, Beater Kirito, who would fight alone, who would fight against opponents just on the threshold of too strong for him, rather than fight safely. Why? Because if he were to die, it wouldn’t inconvenience anyone else, and it’d be just him dying on his own.