According to the latest reports, his 52 year old mother legally purchased the weapons as home defense and told her son where they were and how to use them, etc. Sadly and ironically, she was the first victim to be murdered with her own weapons....
As to you other point, well, a couple of thoughts and you're free to disagree which I suspect you will but hey that`s democracy!
America is very very different place than it was when George Washington spoke those words and the Constitution was designed to be a "living document" that changes with societal evolution.
Additionally, I never have seen a conflict between our Constitution and reasonable, logical and rational gun regulation. The reason in my mind is that the very first sentence of the second amendment reads, "A well regulated Militia, being necessary for....." I interpret this as meaning that regulation of the militia (the people) to keep and bear arms, is within the scope of Constitutionality. I absolutely believe the government has the right and responsibility to regulate the bearing of arms to also uphold its duties to secure life, liberty and the pursuit of happiness and to secure the general welfare. Misuse of firearms is a clear threat to life, liberty the pursuit of happiness and the general welfare.
Like I have said, I am a gun owner and supporter of people`s right to bear arms and am not an anti gun liberal. I just accept and welcome reasonable gun regulation.