1) The card on the art represents new kill indicators from CS2,
in particular the card indicator that appears after 5 kills
2) Agent Sir Bloody Loudmouth Darryl,
the choice of an agent from CS for art is inspired by the Joker
3) The name of the sticker "the killing joke" refers to the name of the
comic book of the same name, the comic book cover of which
inspired me when creating the sticker
4) The phrase "Why so serious?", which appears on a scraped sticker,
refers to the Joker from the movie "The Dark Knight" who said this phrase

This is low violence version of my sticker for Chinese Counter-Strike