The first amendment to the Constitution reads as follows:

Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.

Note, however, that we have modified the freedom of speech in certain cases, the examples given of yelling "Fire!" in a crowded theater; hate speech; incitement to riot.

Note that all of these limitations derive from the danger to other people, violence intentional or unintended.

The second amendment:

A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.

I do not understand why this amendment is considered to be so sacred, so immune to modification, even in cases where violent outcomes are incited, enabled by the presence and use of military (read: mass killing) weapons.

The second amendment should be re-evaluated in the light of the danger of gun useage, especially individual ownership of weapons that enable mass killing and also provision of large ammunition clips.

Yes, it's people who kill. And their behavior needs to be forbidden and their actions no longer enabled. Some weapons are intended only to kill other people.