Abandon sick cat

I saw a very sick cat in my vacation. It was almost dead, very skinny and had a bad skin hillness. I gave him food and fresh water for the time I was there but I know Iit would have 1 week at the most to live. His skin is full of blood from the desease....

Before I leave I think it would be better to put him to sleep (in a confortable way) than to let him die in suffer. What shoud I do? I let him die slowly or help him die,? Can I put a human sleepin pill in his food. Where I am is very poor and there is no vet. Plaese help me with my decision...Poor cat... (sorry for my bad english, I french)

I'm not too sure what I'd do, but maybe putting sleeping tablets in it's food isn't such a bad idea. At least it will go painlessly.