Half of the responses from Tay were from twitter history or "repeat after me". It was a cute little experiment but not really a display of how real AI will behave. Check out this article[1], I think more people should read it before jumping to conclusions.
Tay did not come to logical (or racist) conclusions. It was taught to be anti-social. Humans had to make it that way.
Much like weaponized diseases, AI will just be a very powerful tool that humans can misuse. Hopefully, like nuclear weapons (incredible power, but highly exclusive), AI will be incredibly difficult for the average person to use in a malicious way.
[1] http://smerity.com/articles/2016/tayandyou.html