Betta Fish: Columnaris progression in 6 hours. I’m absolutely heartbroken.

This boy was just delivered three days ago and he's already nearly dead of columnaris. Found that white fluff on his back this morning (pic 1), left for a few hours to get meds and came back to him barely alive (pic 2, dosed with kanaplex since I couldn't find methylene blue), and now he's pretty much dead and completely lost some coloring (pic 3, the sediment is from the betta tea/tannins). I feel so upset/confused/guilty/sad. At what point is it kinder to euthanize? I hate seeing him suffer so much.