I don't agree with what Trump said about violent video games being responsible, although I can actually see someone who is already mentally unstable sat playing something like Call of Duty and obsessing over the guns and perhaps that could feed the obsession a little bit, but no more than any other medium might, like movies, magazines, TV shows or even books. Walmart are damned if they do and damned if they don't.
Why wouldn't Walmart sell guns and ammo though? If they don't then other people will still sell them, and they'll lose money. They're in a highly competitive industry and can't afford to lose money. No one is going to shop there who isn't already because they stop selling guns, and people aren't going to stop shopping there because they do still sell guns, or very few will anyway.