I've two cats, they don't really do outside much, they like a little wander/sit for a couple of hours but are soon back inside scoffing/sleeping/aggrevating each other. The usual routine is I'll leave the back door open for a couple of hours till they've had their fill - this sometimes means the cheekier cats in the area trying to have a nose at their food 
Anyway last night, this little cat comes wandering in. He was in an awful state - weak, couldn't eat, barely drank, didn't make a sound. When I picked him up all I could feel were bones and fur, he immediately just tried to snuggle into the warmth and shut his eyes.
We went straight to the vets with him who diagnosed him as being in renal failure and put him to sleep. I was upset as I'd half convinced myself he'd got trapped somewhere and just needed fattening up a bit, but I wouldn't keep my cats alive with that condition so am comfortable that was the right thing to do in lieu of not knowing who his owners were (no collar/microchip found).
I just feel so guilty - I'm sure he'd come in the night before and I shooed him out before having a good look at him (it was dark). That poor little thing just wanted somewhere warm and comfortable and I made him spend another night outside in pain and incredibly poorly.
Dp doesn't really understand why I'm still a bit teary over it, and tbh I can see his point, but I just feel so sick with guilt over leaving him when I could have got him help sooner.