During a Republican primary debate, Trump said,

In the Middle East we have people chopping the heads off Christians, We have people chopping the heads off many other people. We have things that we have never seen before as a group, we have never seen before…

I would bring back waterboarding. And I would bring back a hell of a lot worse than waterboarding.

In another instance, Trump said that the US should “take out” the families of terrorists.

The Hill – Trump calls for ‘hell of a lot worse than waterboarding’
Chicago Tribune – GOP debate: Trump calls for a lot worse than waterboarding against terrorists
The Wall Street Journal – Trump Reaffirms Support for ‘Worse Than Waterboarding’