Why does the burn septic make the pain worse?!

Burn septic contains Benzalkonium Chloride. (I'll call it BC for short.)

BC is not an antibiotic. It is a biocide. The difference is that an antibiotic is more toxic to bacteria than it is to you, so it should (hopefully!) kill bacteria without killing you.

Biocides kill everything, including your skin cells.

Despite large amounts of marketing to convince you that it does something useful, it is not helpful in most cases because it kills your skin, and there aren't enough bacteria worth killing.

Keep a burn covered with a clean gauze pad so that you don't rub bacteria into it as it heals, but there is little need to try to kill the bacteria that may already be in the burn. Your immune system will take care of that, and any bacteria already in your wound will not be affected much by BC. It is a case of closing the barn doors after the horse has already left the barn. Using BC will just kill more skin, making the burn worse.

The antibiotic kills cells and germs. The killing of your own cells causes you pain. I think it's a small price to pay to keep it clean.