Firstly, I don't give enough of a fuck about what any individual thinks and I certainly don't do background checks on conference speakers.
Secondly, where do you draw the line? Shop workers? Hair dressers? Your taxi driver? Many of those in the country in which I am an immigrant would gladly forcibly deport me and separate me from my family; but I don't run scared enough of them to try to silence them or hide from them.
Also, I think you're being somewhat dramatic here saying he openly called for imprisonment and executions, but please provide sources if I'm wrong.
The thing is, now that you do know someone's views - what are you going to do?
People with abhorrent views should learn that people disagree with them. Otherwise they will continue to spill their toxic poison everywhere.
For sure I don't agree with those people, but they absolutely have the same rights as we do to have their own opinions. Trying to silence them just makes you as bad as they are. We can't change the current state of global discourse, but we can each be more tolerant of each other and that starts with ourselves.
What do you think he is trying to do if not incite violence and hate?