How do you kill a weak chicken peacefully?

I have a chicken and for the past week or two she has not been doing real well.Every time i would come home she would be laying in her tub of food. It was kinda hard getting her out unless I picked her up and put her in the chicken house. But at the beginning of the weak I realized something was really wrong with her. So I isolated her from the other chickens so they would not get sick, just in case it was contagious. She is really weak and cant even stand up (She was limping/hobbling at first). Her right leg is the weakest. She wont eat and the only time I can get her to drink is by forcing her beak in the water, then she will sometimes drink it. She has lost a lot of weight. But I really don't want her to stay like this. I am having a hard time figuring out what I should do? I want her to live but I don't want her to suffer? any Ideas?