I'm so sorry you lost him, it's so disheartening when you've done everything you could to help
There are a multitude of ailments that hatchlings can come with that they acquire long before we improve their lives. Most are kept in such deplorable conditions in flea markets and the like that it's hard to tell what could be wrong. Also, at their tiny size it's hard to diagnose them.
The soft shell could be a sign of MBD which would be from the lack of proper lighting and calcium he got before you took him home- it would take a while for MBD to develop, it's a serious lack of calcium, so sadly perhaps Monster just was too severely affected before the calcium you and the vet gave him could help. It could just be that he had too many things going wrong and not enough strength to fight them.
I really think you did everything in your power to help him. It's devastating to lose a pet, but don't beat yourself up over it, you gave him a wonderful life and you'll continue to give Murphy one.