Sure, you personally wouldn't comply to that, but there are definitely people out there that would, and will.
For the sake of reality, I'll agree that there is at least one person in the world who would willingly loan their sketchy-ass friend firearms. But that's not even the point. Giving high-risk individuals a legal route to gun ownership makes it far easier for them to acquire guns. Depriving them of that route will naturally lower crimes of that nature.
I'm not gonna say that it's capable of making a dent in the overall percentage of deaths by gun violence in the country, but it'll save some lives. And honestly, what's the downside here that I'm supposed to bet against? A schizophrenic guy has to purchase an alarm system to protect his home instead of an AK-47? What a tragedy.
No matter how many gun regulations are put up, people will still have access to firearms.
It's something you really can't stop and just putting down more and more regulations would just make it worse and eventually override the 2nd amendment, which is something I like to have y'know.
People owning firearms isn't bad, and people owning fertilizer isn't bad either. The vast majority of people aren't opposed to letting people own guns for self-defense and entertainment, just like the vast majority of people aren't opposed to the sale of fertilizer.
If creating common sense regulations for firearms is a 'slippery slope' to a total ban on firearms, then the restrictions we've placed on fertilizer are eventually going to lead to a nationwide fertilizer ban that will deprive everyone's rights to home gardening.
Did it stop the Boston Bomber?
I'm unimpressed by the fact that you're giving me a specific case when I've already addressed why laws aren't always written with the end goal of entirely eradicating a certain type of crime.
Also, he didn't even use fertilizer IIRC