That image was attached by email without attribution to an image originator. Well, let's see good guy bad guy, each with a gun; bad guy could zap good guy, so already a problem has arisen. A bad guy with a gun could stop a bad guy with a gun. That's certain, and it obviates "the only" part of the image/premise. A worse bad guy at that could be the one with a gun to stop the original armed bad man. Or with luck - less bad a guy, with a gun, but still bad as sin ... he could be the one to zap the original first and worse bad guy. There are a range of outcomes. Add the 'Ugly" -- Eli Wallach in the film, and then those pairing round robin possibilities besides good/bad mixes simply abound.
Still the premise, need a good guy with a gun, what about with a tractor or a truck, run over the bad guy with a gun? Leave that bad-actor devil with tread marks up the middle ...? Stop him cold.
An army could stop an armed bad guy, outnumber and surround, just as with Custer.
From behind, a stealthily moving ninja could top a bad guy with a gun, surprise making up for superior weaponry; surprise being the lesson of Valley Forge and the Hessians. So the premise of the image, as cutesy schlock as it is, is not well grounded.
Finally, that NRA stuff arguably held for the ending of Fight Club, although traditional simplistic-enough-to-appeal-to-NRA-types notions of good guy -vs- bad guy scenarios do not exactly fit Fight Club's ending sequences.
How about this: an existential man with a gun; could he stop just about anybody else equally armed? What about a secular humanist with a gun? An abortion provider with a gun, you tell me, good guy or bad guy? An anti-abortion would-be abortion clinic murderer - with a gun - he could be stopped by an armed and ready provider; (readers to provide personalized "good" and "bad" labeling).