Criminals break laws.
The vast majority of them are not criminals until the moment they pull the trigger.
Your two examples (out of how many thousands of gun crimes per year?) do not excuse anything, they just make a case for background checks being broad enough to include family and friends (would you sell a man a gun if you knew his brother was a drug dealer?).
No change in the law will prevent every crime, but there need to be three simultaneous and active strands to saving lives:
1. Proper gun controls> proper background checks
> proper "cooling off" time
> having to undergo proper training before taking possession
> random check of gun-storage by trained officials
2. Top-down action.> no more platitudes from elected officials. "Thoughts and prayers" should be a banned phrase.
> get pro-gun lobby groups away from those in power. If you need NRA money and influence to get or stay in power, you are not worthy of being elected.
3. Positive media action.> Cover more shootings in depth, not just the "cool" ones. Shove the whole gun-crime scene down the throats of media consumers.
> Portrayal of guns in faction should be negative, or at least realistic - think of the thousands of rounds fired in an episode of A-Team with not a single scratch, and everybody goes home alive and smiling.
> Exploit the moral high ground. Drink-driving used to be a major problem in the UK. It was considered perfectly fine to spend the night in the pub and then driving home, even after drink/drive laws were tightened, but media portrayal didn't just make it illegal to drive drunk, it made it immoral.
> Stop portraying countries with proper gun controls as weak, cowardly or "liberal".*
I'm done arguing this. As far as I am concerned, there is no excuse for avoiding better gun control at every level of government. The only reasons to continue with the status quo are selfishness and cowardice.
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* Seriously, genuine liberalism is the founding principal of the USA (it was the philosophy used to persuade the people to turn to armed revolt!), and personal liberalism is the only way any civilised society can progress.
Those here claiming they prefer personal freedom, that they distrust Those In Power, you are liberals!
https://en.oxforddictionaries.com/definition/liberal