What would happen if a person ruled the world single handedly?
That's not possible.
Why would anyone waste their time contemplating the impossible when there are so many more pressing matters at hand to attend to?
That has tempted tyrants and other people in the past, because then at least you could have consistency and coordination in policy. When you split the power up among people you get dissension, and loss of coordination and cooperation. It weakens a plan usually. So a single ruler could get the trains running on time, but the problem is, of course, that if they have unchecked power they can go off in a bad direction, like hitler for example.
They would fail miserably because "absolute power corrupts absolutely".
It depends on how he rules, leadership and personality
they would probably die doing it since theyre not getting any help
Depends on the person, how they ruled the world and how everyone else reacted.
...everybody would become happy - let me do that.
Check out Alexander the Great. He ruled the world single handedly.