Guilty conscience

This morning I went into the kitchen to get breakfast and I heard a cat crying outside. I opened the back door to see what was going on and a little orange kitten came streaking into the house through the open door! S-Boogie was extremely delighted, and I thought he was pretty cute too. He was obviously wild, hungry, and desperate for a home (but not for human contact, which I learned when I tried to pick him up). I love animals and was very tempted to keep him around, but we had several problems: Master Fob is allergic to animal hair, the cat was not potty trained and would pee all over the house, also it was obviously not neutered or immunized, and I was worried that it might pass some disease on to S-Boogie. So I took him out back and tried to put him down at the edge of the parking lot by the grass. But, a few minutes later he was outside crying by our window (it's at ground level) and trying to get me to let him in. I called the animal shelter and got a recording stating that they are closed for the weekend. After a while I didn't hear anything and figured he went away. A few hours later he was still there when we left to go somewhere and I felt horrible all over again. By the time we got home he was nowhere to be seen, so I hope that he's all right. I feel terrible, but there wasn't much I could do for the cat. If we had a garage or a yard I would have lured him in there, but we can't keep him in the house. I just feel sad and hope he gets taken in by someone who can help him. Or maybe he'll come back on Monday and we can take him to the shelter (yeah I know they'll probably gas him, but it's better than starving to death or getting run over). Sorry kitty.