Death
Klaatu is killed by humans in the final part of the film. Ironically, he knows if he dies that Gort will destroy earth and has planned to have Helen program him to resurrect Klaatu with specific words spoken to the robot in order to give humans another chance.
Gort
Klaatu is in charge of Gort and commands the robot. Ironically, we learn that Gort is part of an indestructible force of robots that will destroy any planet that doesn't act peacefully. Therefore Gort, while being created to keep peace, does so by committing violence.
Led
Klaatu is led around Washington D.C. by Bobby, Helen's son. The irony in this is that Klaatu is able to see the earth through the eyes of a child--with innocence and having an outlook on the world that is from an unobscured place of hatred, but rather from simplicity and love.
Diamond
Bobby sees Klaatu enter the spaceship, and Tom and Helen don't believe him. Ironically, they learn that Bobby is telling the truth once they have the diamond he finds examined by a jeweler only to find out that this diamond doesn't exist on earth.
Second Chance
Klaatu gives the people of earth a second chance to live in peace. The irony of this is that Klaatu was just killed by humans, and it was only be being resurrected that he's able to stop Gort from destroying the planet and give them a second chance.
