I would steal the cat 
I once found a really thin cat, fleas, scabby ears etc sitting at the bottom of my garden and although he had a collar on (with no identifying information) he looked poorly and homeless. I took him to the vets to get microchipped but there was no chip.
I took him back with me, took his collar off as it was so tight it was rubbing away his fur and gave him some food and water.
A few days later he reappeared with a new collar on so I knew he must belong to someone. I put another collar one (to get the owners attention) and attached a message asking them to call me.
A few hours later the owner phoned me, they lived on the same street. As we got talking I felt pretty incensed actually that they didn't care what state the cat was in. They also told me the cat isn't allowed inside the house because it upsets the dog 
The cat continued to visit and I fed him. Because it was winter and freezing I bought him a kennel with fleece blankets and put it in our garden so he had somewhere warm to go seeing as his owners didn't let him in.
As the months passed I took on more responsibility for the cat, I even took him to the vets and paid for his appointment and treatment when he had a nasty abscess under his eye. I had previously mentioned this to his owners who had proceeded to do nothing about it.
I would text the owners frequently to let them know the cat was at mine or it was unwell etc and they didn't care.
One day I thought "fu*k it" and I let the cat into the house where he made himself quite at home. A few days later I took him to the vets and had him microchipped with my details.
The cat practically never left and I never text his owners again. Not surprisingly they never text me to see if he was with me seeing as they wouldn't have seen him for days.
I feel absolutely no guilt whatsoever.
Animals need kindness and if you think the cat is being neglected or mistreated then absolutely show it some love.
There may be some very much loved cats out there with medical reasons for looking scrawny, dirty and having fleas
but the likelihood is more that they have owners who just don't care about it.