So, the Punisher is a comic book character that was formerly a villain but became somewhat of an anti-hero in the 80's, when vigilante heroes were very popular due to right-wing sentiment that lawful "red tape" meant bad guys ran amok because they had...y'know...human rights.

Unlike a huge majority of Marvel characters, Frank Castle not only disregards that pesky "due process" and all that jazz, he straight-up goes and kills the bad guys , like gangs, mafia, and so-on.

Military guys started adopting the skull logo for intimidation purposes, and the cops who think they're military thanks to crap like the "war on drugs" and whatever, adopted a "thin blue line" version of the logo, which basically screams:

"I have fantasies of violence and would probably take the opportunity to 'clean up the town' by just murdering people I thought were bad without those pesky 'laws' and 'rights' in the way."

Or

"I support cops who would do that."

It's worth noting the authors of the comic book character included an issue where cops wanted a selfie and clearly idolize him, and Frank Castle "The Punisher", tells them off because they're supposed to uphold the law , and they should be looking up to Captain America instead.

So, you'll see these bumper stickers proudly displayed (with many different variants), and they scream the same thing: "I'm a self-centered, self-righteous jerkface who hates everyone who isn't me, I wish someone would just kill everybody I'm pretty sure is bad, no questions asked."

It's since gone even further through the MAGA-filter like everything else once-even-vaguely-conservative.

--Source: Am Damn American, but don't use so broad a brush. We're screaming right along with you. :)