Why leftists insist that words are violence and ideas are dangerous

This conclusion isn't something Milo Yiannopoulos invented.  Nono.  This was an explicit agenda set forth by famous cultural Marxist predecessor Jean-Paul Sartre — to redefine ideas and words that "his team" (now we understand it's the modern cultural Marxists) considers disagreeable as "violence", creating enough of a climate of moral hatred for those ideas and words, that allows them to retaliate violently against anyone who utters those ideas and words, and make that retaliation appear morally justified. which would ultimately allow them to completely and totally dominate thought, forever.

Quoted from Thomas Sowell:

France's Jean-Paul Sartre has been credited — if that is the word — with calling social conditions he didn't like "violence," as a prelude to justifying real violence as a response to those conditions. Sartre's American imitators have used the same verbal tactic to justify ghetto riots.

From the Stanford encyclopedia of philosophy:

Sartre came to recognize how the economic conditions the political in the sense that material scarcity, as both Ricardo and Marx insisted, determines our social relations. In Sartre's reading, scarcity emerges as the source of structural and personal violence in human history as we know it. It follows, he believes, that liberation from such violence will come only through the counter violence of revolution and the advent of a “socialism of abundance.”

Wed see here, not only the economic Marxist connection with the cultural Marxist agenda, but the deliberate intention Sartre had to respond — and have people respond — to non-violence with actual physical violence.

Which we are seeing happen now.  The belief that mere words are equivalent to physical violence is an actual matter of United Nations policymaking now:

In the United States, we are beginning to see that notion of "ideas / words as violence" dawn onto the populace.  This is how society looks like, in a world where people actually believe such nonsense:

Who will get the hit in the face next for "opining wrongly"?  I do not know.  The next person could be you.