The motivation for most slayers is vengeance, while rengoku cares more about keeping people safe. The only thing i found odd was during the train when everyone was asleep, Rengoku’s soul was depicted as a scorched wasteland, from what we’ve seen of him i would have expected it to be a more positive place, maybe not the same as tanjiro but …
https://www.reddit.com/r/DemonSlayerAnime/comments/14r1ki5/shit_why_is_rengokus_death_so_sad/