Breh...the 2nd amendment is...an AMENDMENT to the Constitution. Jesus Christ. The right to bear arms was not granted by the original constitution. Laws can be made and changed to fit the needs of a time. You're talking about laws as if Moses descended from the mountain top to deliver them from God.
It's not a guarantee that someone who wants to kill people at 18 would follow-through at 21...due to both biology as well as socialization. I'm shocked I need to explain this to you.
The crazy thing about this "oh well he would've found another weapon" argument...if you don't think a 30 round mag makes a difference vs. a 12 round mag in terms of how efficiently you can dispense violence, I don't know what to tell you. You just sound uneducated on guns or disingenuous.
AR-15s let you engage more targets at a longer distance with more lethality. The entire approach the military has taken with these type of weapons is to have guns and ammunition that can allow a person to put an overwhelming, seemingly unrelenting amount of bullets at a target from a distance.
Look at the Vegas shooting. You're telling me dude would have killed 60 people with a pistol from his hotel room? Or are you saying he was going to go down there and start John Wicking it on the streets?
I can guarantee you that if everyone who had a mass shooting racking up 20+ kills needed to instead build a bomb, be a sword wielding ninja, or John Wick it, a lot of those people killed would still be here today. Not all, but many. If not most.
People are lazy and impulsive. You'll curb a lot of casual (and homicidal) interest in weapons by just putting red tape there. This isn't rocket science.