I'm thinking of charging for a re-write after I did the initial work for free
Is there a reason you would do this?
I couldn't guarantee that I'd done a good-enough job the first time
It was good of you to give your services away for free the first time, but you really should have given the best possible effort.  If you think you can do better, though, it's find to charge for more of your services.
I didn't think I'd get so much shit for pricing my value.
You have every right to charge for your services. People should respect your work and know that not everything can be free. It might help to have a pricing guide for future references.