People keep saying that if guns were made illegal only criminals would have guns. While this is technically true, it is misleading. Criminals can obtain guns much more easily because so many guns are sold legally. If guns were made illegal, it would be much harder to obtain them, fewer would be produced, etc. A large portion of guns used by criminals are stolen from people who bought them legally. If people aren't buying guns legally anymore, this avenue would be lost.
And you have to draw the line somewhere. The "it's a slippery slope" argument can go both ways, really. If it's a constitutionally protected right for people to be able to own any gun they want, regardless of the danger involved, what about tanks? Nukes? Is the right to bear nuclear arms also a constitutionally protected right? Are we like the Nazis if we don't believe in it? I really don't think ordinary citizens need 30 bullets in a magazine. It's true that this is exactly why he was able to kill so many. He was tackled as soon as he had to reload. And yes, we need to be more careful not to sell guns to crazy people, but a lot of crazy people haven't been diagnosed yet.
And I think that Nazi thing is a horrible analogy. So the Nazis took away people's guns. So what? Most of Europe has banned private ownership of guns but they aren't all Nazis. What point are people trying to make here? That the US will become the next Nazi Germany if we ban guns? That's certainly what the implication sounds like. Come on, now. If you're trying to make the point that private gun ownership would've prevented Hitler's rise to power, I think that's an overstatement. It might have made it a little more difficult, but I think it was inevitable considering what was going on at that time and place.