And yes I know it's fucking stupid what Karofsky did. Also it was fucking stupid that Kurt didn't tell anybody about the threat. I would chalk it up to them being stupid teens not knowing what to do in these situations.

So, you're implying that Karofsky planned that whole apology, even though Kurt unintentionally instigated it. Plus you should know that Karofsky is struggling with his sexuality plus fully regretting all the shit he did to Kurt. Karofsky cracking under the pressure was bound to happen eventually.