His tweet about Ilhan Omar disengenuously implied that she was minimising the 9/11 tragedy.
Her speech (if you see it in full) was making the point that Muslims should not be judged - as an entire people - by the actions of terrorists. It's heavily ironic that the Trump retweet underlines Omar's point perfectly.
Since then, Omar has received an increase in the number of death threats she's received. Note I said 'increase' because she's received plenty already for the crime if being a female Muslim leftists who is outspoken.
Death threats on the Internet might be perceived as crazed tirades by a bunch of saddos but then what were the words of the Christchurch shooter? The rhetoric of politicians pushes the bar a little further and its pushed further still by 'edgy' online posters until...
Obviously Donald Trump cannot be held directly responsible for political violence because he doesn't directly advocate it. Well, he sometimes says he'll pay the legal fees. However, he should still show responsibility for his rhetoric and the real world consequences that such words can bring. Though obviously the idea of Trump and responsibility enters the realm of science fiction.