Free speech has barriers. Like all freedom, if the freedom of one minimizes the freedom of others; or if one specific freedom minimizes another freedom, a fair balance has to be uphold. In the case of freedom of speech, it is such a fair balance to ban incitement of the people in favor of the other forms of freedoms like the right to exist without being subject to such forms of hatred.

France, Germany and many other seem to agree in that point. Those tweets exactly cross the barrier, and the author has to be prosecuted.