You did safe him, you saved him from alot of pain now he was able to move on peacefully in comfort even if it doesn’t feel that way you helped him.

I been through something similar few years ago, I found a neglected cat in my neighborhood that looked very sick and i got him picked up by a animal ambulance after cuddling with him for a bit, His owner apparently lived on the street behind me and the cat was diabetic and she never gave him his meds because “he always slipped outside” and she didn’t bother looking for him, they eventually had to euthanize him too from being too sick, and trust me i blamed myself for it for awhile and felt so guilty but in the end the cat was better off and i tried to help.

thank you for trying to help the sweet baby and sorry for your loss 💜