The Cato Institute has published a study that determines the likelihood of being killed by foreign-born terrorists.
As you can see, there have been only three deaths in the US in the last 40 years attributed to terrorists who entered country as refugees. That's an average of 0.075 deaths per year, or a 1 in (roughly) 3.6 billion chance.
So here's a short list of things that are more likely to kill you than a refugee if you live in the United States: