On FOX: 
“He’s a killer, though,” Mr. O’Reilly said. “Putin’s a killer.”
“A lot of killers. We’ve got a lot of killers,” Mr. Trump responded. “What – you think our country is so innocent?”

Didn't watch this exchange.  Perhaps O'Reilly pressed the point.  If not, he should have: 
O'Reilly: "So, it's a race to the bottom?  Are you saying that the American leader should emulate the Russian leader?  How does that make America Great Again?"

It's called baiting the bear. And I would be unapologetic in doing it, because, in doing so, the interviewer would be doing the country a service. Expose the man's character again, and again, and again. The US doesn't need sheepish, smiling "aw shucks" media types fawning over the President.  If he puts himself out there (and he will, repeatedly, it's in his nature)--fair game.