We gotta remember that Kim isn't very bright, isn't very brave, and isn't very well informed. I don't think he has ever visited the US. Does he even speak or read English? His ideas of what we might do are formed from the Communist propaganda he must have been raised on, and eight years of Obama's opaque weasel words and red lines.
It's right and proper for Trump to tell him "You nuke anyone and we will nuke you down to bedrock," Kim may not have understood this before Trump said it. Even if Kim didn't catch on this time, I bet there are some people in the regime that did.
It will make them cautious. That is a good thing.