To me, the founders made two fundamental mistakes when writing the Constitution:
1. The Electoral College. Our voting system was never intended to allow someone with fewer votes to become president. That is really an anathema to the whole system and intent of a democratic Republic.
It is like a computer glitch, a coding error. That we have gotten that result now twice in 20 years is proof of that. Electing the "wrong guy" (be they a Republican or Democrat) is just a bad by-product of a poorly-worded clause in the Constitution.
2. The 2nd Amendment: Whatever you say, other countries that don't allow private gun ownership like we do don't have mass killings,
just ask Australia. And with technology and killpower getting ever better, these events are only going to get ever worse.