Mareks virus can cause them to lose control of everything eventually, including the ability to eat. I have heard if you can keep it tube fed, it can live through the disease and recover, but become a carrier. I think if the only problem your chick has is with it's legs, it still might be a tendon problem or some other deformity. If it's losing control of other parts, then a bigger problem. Sometimes Mareks will give them an irregular pupil and change eye color.Not sideways like that. one straight out front and one straight back.
I think you are right to cull. Is it a frizzle?