I lost a chick today too--no idea what happened to it or why it died--so I really feel your pain now. I'm keeping an extra-close eye on the rest of my babies just in case any of them start showing signs of distress, especially since I have no idea what caused the chick to die. I'm thinking of...
I'm so sorry for you loss! It doesn't sound like you've done anything "wrong"--according to Storey's Guide, spraddle leg tends to happen when you use newspaper or slick paper rather than something textured like paper towels. I wonder if the limping was just the first sign of a deeper problem...