I don't think you know what those words mean? Generally when people refer to "decompiling", they're talking about reverse-engineering a program to arrive at equivalent source code. (A "disassembly" is similar, but in that case the product is assembly language with useful notations.)
So, no, decompiling does not "require a Source", but it's a nonsense question.
That's generally called "cracking". That has nothing to do with "decompiling".
In any case, is this a 3DS game you're talking about? Because if it isn't, why are you posting in the 3DS thread?