The United States has become a very violent society in the last five years. People have decided that violence and protest is suddenly a way to deal with things. Trump should condemn the treats directed at Carrier, because Trump has been directly involved with working with them on their business dealings.
Those in power should always condemn violence, regardless of whether those who are targeted are those in the wrong or not. A civilised society deals with problems in democratic ways, not with fists, especially one who the rest of the world look to as leaders. If those at the top act like animals, then where will we be?
Yes, Trump should condemn threats of violence directed at the Carrier Union leader. He should always condemn any act of violence. That's part of being President, or president-elect in this case. On top of that, it is Trump's supporters who are making most of the threats. He is the one person best suited to calm the threats. It's his responsibility, but not one he is likely to take.
Trump needs to act as a leader. Part of this means that no matter what, he needs to make sure that people understand that violence is not the answer. He needs to step up and show that he does not condone violence towards anyone, not just in this particular situation.