As someone whose academic work revolves around media effects, I can agree with many of your points, in particular that exposure to media violence desensitizes one to real-world violence. However, lasting effects are mostly seen among young males (and that’s with a lot of other factors at play), and many studies of it have measured sympathy rather than empathy — an important distinction. The real takeaway is that the desensitization is a defense mechanism, and usually, its effects are temporary. Therefore, I can’t really agree with your implication that violent media perpetuates a culture of violence. I would note that violent entertainment dates back long before movies even existed. Even after bloodsport went out of vogue, circus began with battle reenactment and plays featured stage combat.
Perhaps, in a world so fraught with violence that we also hear about everyday in our constant streams of information, movies like Deadpool are an attempt to achieve that defense mechanism. People are willing to watch fantastical violence because of its unreal-ness, while the news is too much to bear. Cinema has always had an escapist function, and the jury is still out on how much exposure to violent (or any type of) media impairs one’s ability to discern fantasy from reality. Consider the influx of post-apocalyptic dramas, anti-heroes, and stories about tyrannical government that are currently popular. Are they an artifact of a violent society? Or are they created because people want to comprehend, if only artificially, what they would do when presented with looming environmental crises, government corruption, assaults on reproductive rights, or an epidemic of gun violence?