I had a similar dilemma a few weeks ago, but the moggy was quite motionless on the kerd at the side of the road I went up to it and there was no blood, so I summised it had just peacefully passed away. I step over it and left it, but felt I should have done something, but didn't know what.
I passed the same spot a fortnight later, and the same deaf old puss was alive and well and after my assistance in trying to get into its own. Needless to say, I again wandered on without helping the fleabag.