Absolutely, AnneClaire is right. Some of them just have problems that will not allow the kit to survive, no matter what you do.
You are the one who is there and can see the actual condition of the kit, how it is acting, etc.
I have to confess, I have not been able to bring myself to cull a kit yet, though I've had very few opportunities to consider it. Only one, actually, now that I think about it. I tend to supplement runts and kits that aren't getting enough, just because I can't stand not to. :roll: All have grown up except one. That one never grew, and started getting weaker. At day 13, I withheld one meal, and it died! This confirmed my suspicions rather quickly that it had some other abnormality at work.
Supplementing vs. culling... neither is wrong. You know the kit, and you know whether supplementing might help... and you already don't think it would at this point. The next step is up to you, and you've got plenty of support here, whichever option you choose.
I'm not sure about the fur... I did have patchy, scruffy fur in a kit that wasn't fed well once. It had another issue, though: It had managed to get out of the cage and onto the chute, or onto the dirt, several times. It ended up with some fungal skin thing that I treated with athlete's foot cream and hydrocortisone. With supplementation, he eventually got all his fur, though it stayed scruffy until he was about 8 weeks old.