Die Hard/WMG

John Maclane has Saiyajin Blood in his veins.

Well, it makes sense if you consider the following statements.

"The stronger the enemy, the stronger we become."
"When brought back to life from the brink of death, our battle prowess multiplies exponentially."
"You Saiyjains are a warrior race at heart, yes? You love battle above all else!! The stroner the opponent, the greater the joy."

Now, let's take a look at Johnny.

• He was basically a glorfied shopping mall cop. Then along came terrorist veterans of the fiercest battlefields of 1980's Earth.
  • He was absolutely shredded into a bloody mess by these bastards, barely coming out in one piece after each confrontation.
    • And yet he not only (barely) survived, but DEFEATED all of them.

• 3 years later, the hero of Nakatomi Towers is able to dispatch multiple veterans of the Persian Gulf War completely unscathed. It takes a master Martial Artist like Colonel Stuart to actually bring him down, and even then only barely.

• Another 3 years later: FIVE Veteran soldiers + Jonnny in an elevator = 5 Dead Veteran Soldiers and one PERFECTLY FINE Johnny.

• FIFTEEN years later? Kills a helicopter bristling with heavy-artillery by ramming a car into it.

Johnny faces progressively stronger opponents, gets shredded into pieces each time, somehow comes back alive and is stronger afterwards.

Just in time to face the next EVEN MORE powerful opponent.

And as his Saiyajin nature awakens by Die Harder the ONLY times he's shown to be happy is when he is in battle brutally killing enemies: He is otherwise bitter angry and morose ALL the time, like Vegeta or ANY Saiyajin who is deprived of a strong enemy to fight for extended periods of time.