Looks like cutworm to me. Cutworm chews through the bottom of the stem and then feeds on the plant roots. The ones that are cut off will not do anything now and that cutworm is directly under the plant about where the roots are. Dig down and remove it and step on it. There will only be one but if you leave it it will move on to surrounding plants.

Nah I'll just sterilize the whole thing with boiling water .

The boiling water would work if it was a surface problem but it won't be boiling when it reaches the cutworm. The roots will be about 4 to 6 inches under the soil and the water will be cooled off percolating through the soil. Cutworms are very easy to dig up with a shovel. Dig right beside the plant and throw the dirt on a cardboard or tarp. Sift through it and you'll find a grub like cutworm. It's really the easiest way to get rid of them and know they won't be destroying any more plants

I use 2 inch cardboard cylinders cut from toilet paper tubes to protect my seedlings. The cutworms don't like to crawl over them, and they're compostable. ( Hint from Pat Welsh.)