no backstory. the joke is basically that op is an evil person but has changed now just wants love which is supposed to be humorous, their title exaggerates the joke by claiming they committed war crimes in the past but are still lovable

Little bit of backstory - it’s from a popular cartoon (She-Ra) where the catgirl pictured is one of the villains initially. The running joke in the fandom is that she has committed many war crimes.