There is evidence that high doses of antioxidants can make cancer worse, probably because cancer cells are under oxidative stress.

https://www.cancer.gov/news-events/cancer-currents-blog/2015...

https://pubmed.ncbi.nlm.nih.gov/8127329/

Vitamin C in large doses becomes a pro-oxidant because it reduces metal ions, and they in turn then become catalysts for oxidative reactions or even directly oxidize stuff.