Maybe Captain has some affection for him thinking of his grandson -despite not trusting him- thus prompting him not to backstab him after getting to know him at least a bit better in the lastest chapters.
Something about when Higanjima becomes a trap maze, a boss fight, a dungeon crawler or a mix of these is really entertaining to read.
I like the idea in the comments about a videogame version for it, it would indeed work really well overall.