It's a fair reflection of an opponent, not an endorsement for crimes. Trump has been warning about North Korea's nuclear weapons and development for months.
I agree that Trump wasn't endorsing or praising him, but Trump's statement was not a "fair reflection" of Kim Jong Un. It was not "amazing" that he was able to seize control - North Koreans (including, to some extent, even the generals) have been indoctrinated over the past 60+ years to believe that Kim Jong Un's grandfather, father, and now himself are, essentially, part-divine. Couple that with a few friendly/loyal/influential generals and some standard tyrant methods of consolidating power (read: killing off rivals), and it's not at all surprising or impressive or credit-worthy that he was able to do what he did at a young age. So, no, there's nothing offensive in what a Trump said, but it is a pretty shallow, poor understanding of the situation in NK.
Bullies often admire thugs.
Working into the wee hours to spin the latest babbling of The Hero? Does he not understand that all the world is not "Saturday Night Live"?