The worst mass shooters in the US were all murderous suicides. People who had concluded to end their lives by their own hands. Whether they first thought murder as many people as possible, then about how that would be best achieved if suicide at the end is an option; or whether they first thought suicide and only then 'figgered' might as well kill a bunch of people on my way out, we can only guess. We know they concluded to end their lives by their own hands (including 'suicide by cop').
Kamikazes killed as many as possible while killing themselves. The worst mass shooters in the US were like Kamikazes in this regard. 9/11 was Kamikazes.
The operative clause in the Second Amendment is "the right of the people to keep and bear arms shall not be infringed."
It means, and it's spoken directly to the Federal government, which has since officially incorporated this Amendment against all the states, it means "don't meddle in gun markets," and it means that we can't be harassed by police for carrying weapons, and it means that we are all authorized to carry guns everywhere we go, with exceptions that apply to LEOs only. Meaning, if police officers can't carry guns, then and only then is it permitted to forbid any American civilian from carrying guns also.
The very worst things that humans have ever done to humans is associated with cases where the right of the people to keep and bear arms has been infringed, where gun markets were meddled in, where police harassed civilian people with guns and civilian people who wanted guns, where only police and military were authorized to carry guns anywhere and civilians are restricted from or forbidden from doing so.
The US service rifle is the M-16 (with various suffixes), and the US service carbine is the M-4 (also with different suffixes). The US service rifle used to be the M-14, and before that, during WWII, the US service rifle was the M-1 Garand. The US service carbine was the M-1 Carbine then, and though they are named similarly, the Garand and the Carbine are not different versions of the same gun, they did not even share ammunition like the M-16 and M-4 do today.
During and after WWII Americans could buy as many M-1 Garands or M-1 Carbines as they wanted to, and could carry them wherever they wanted, with few exceptions.
When you examine data, gun data and murder data is unrelated. If you roll dice and flip a coin at the same time, there is no pattern between when you flip heads or tails and which numbers come up on the dice. There is not pattern. That's what gun data and murder data do. There is no pattern there either, and that definitively does mean that gun control does not improve murder rates, and that also having civilians awash in plenty of guns will also not worsen murder rates---both those things are absolutely true.