Why are the dark angels so prone to killing guardsmen after battle?

So, as I’ve learned more about the 1st legion, I discovered that apparently they will, after battle, slaughter surviving guardsmen. This is apparently more likely to happen after battles against the forces of chaos, or The Fallen.

Is this just a meme, or do they actually do this? Why?? They don’t seem as ‘guardsmen are fodder’ as the Black Templars or Carcharodons or all the other ‘edgy’ chapters.