We live in a largely GOP structured country. The weapons of war that they insist must be easy to obtain are FAR too easy and young men who live a video "first person shooter" life make those weapons their first choice in acting out the mayhem they have practiced for so long.
For over 30 years, conservatives have hammered the message that liberals are dedicated to destroying the country, and of course in recent years there have been more on the left that respond to this rhetoric in kind.
The former president has built his image on violent and hateful words, has encouraged violent responses to a free and fair election that he lost...he is the last person who can legitimately claim surprise when violence comes back to his door.