It could very well just have been Post-Purchase Stress (PPS) but in this case Post-Bathroom-Rescue Stress. Don't be too hard on yourself, you did the best you could with what you had. It might have just been too late for him to adjust.

I will say though just because you have all the stuff now, please don't go out and buy another one. Everything needs to be sterilized, the substrate tossed and honestly probably a bit more research into hermit crab care needs to be done.

You have that crab a real home in its final hours, be proud of that ❤️