Irony
Interesting that the time traveler was not a fan of war. He seemed repulsed by it even when speaking of war and clearly had no problem leaving his own time period. Yet his first instinct in the future was (is) to teach the people to fight and destroy the underground dwellers- even though they are also considered humans. Of course one can argue self-defense but it's still warlike.
The same concept is in Planet of the Apes. Heston's character rejects the human race then becomes it's sole defender.
I wonder if this theme is intentional or by accident in these movies.