Why does there need to be a God?

[Fake Messiah]

But as an adult it is absurd, that even then, you need to be told not to murder or rape or steal. 

Yeah, that is called Argument from adverse consequences

X is true because if people did not accept X as being true then there would be negative consequences.

X is false because if people did not accept X as being false, then there would be negative consequences.

X is true because accepting that X is true has positive consequences.

X is false because accepting that X is false has positive consequences.

Or as Roman historian Polybius said/ wrote:

Since the masses of the people are inconstant, full of unruly desires, passionate, and reckless of consequences, they must be filled with fears to keep them in order. The ancients did well, therefore, to invent gods, and the belief in punishment after death.