There wouldn't be a world war. What are you even saying??? There's no other nation involved.
Killing the man could potentially save countless lives, avoid a potential civil war at the least and maybe even avoid major international conflicts where the US would actually associate with rogue countries and leaders and leave it's regular allies.
Europeans are actually working towards a possible scenario where Trump gets elected and stops supporting them in favor of Russia!
If anything, Trump could be the the potential spark to start world war 3!