Killua is likely to be completely devastated. He might go into a state of shock at first, not believing that Gon is really gone. Then he could become very angry, either at himself for not being able to protect Gon or at the situation that led to Gon's death.
Most BSD light novels often have complex characters. Since they are based on literary figures, they bring in the depth and complexity of those original personas. For example, a character might have the same traits as a famous writer but with a unique twist for the BSD universe.
Gon in fanfiction can be very single - minded in his pursuit of goals, just like in the original. He might be a bit more naive at times, which can lead to interesting plot twists. Killua is often portrayed as having a complex emotional side. He struggles with his past as an assassin and his loyalty to Gon. His cool exterior may crack in some fanfictions, showing his true feelings.
The appeal of all BSD novels lies in their ability to create immersive worlds. They draw readers in with their unique concepts and engaging storylines. For instance, the way they handle character relationships is very captivating. The conflicts and reconciliations between characters make for a great read.
Honestly, there's no definite answer on when the BSD manga will end. It all comes down to various factors like the author's inspiration, the popularity of the series, and the publisher's decisions.