If I use a decompiler to get the source of a known C++ .exe botnet bin will the program execute during the "decompile" ?
What decompiler should I use?
No. Decompilation treats the binary as data and does not execute any of the code.
It is like looking at sheet music to determine the structure of the music. The music is not played or heard.
The disassembler or decompiler to use depends on the architecture of the executable. Since you mention .exe that suggests it is MS Windows or Vax/VMS. Search for one suitable for your environment.