Few points worth of noting in this kind of conversation, that apply to any general military force organisation:
1) Through the ages there has been a force to deal with the ones that wont follow the orders of the general (this could go for a very deep, multi branch example, so i leave it at that). At the modern military (in our dull world, not the awesome 40k), we have Military Police. Each nation with slightly different roles etc, but that is the force to hunt for deserters and capture them. Ofcourse in such a sivilised times they hardly act as executioners for the ones not in line, but thats not the point here - the point is that such a force outside of normal organisation (that still takes part in action at some armies) exists.
2) To have a controlling force to be effective, it needs to have atleast equal measure of force at its use.
So, these in mind. I giggled myself for the idea of planetary police force arresting Angron for example Good luck trying to do that with tear gas, batons, and water guns..
And even the imperial guard couldn't handle the legion if it was forced to be put down. Or it would cause so much damage to a sector forces that a whole segmentum could be easily crippled. ..And just how many lasgun shots (on average) you would need to kill over 100k space marines (firing and charging at you..)
So, a force that naturally is inwards attuned, stays in their own group and are fiercely loyal to their commander was created. To be out of the influence of others that could need the punishment. Also, a group ferocious and fearless enough to get the job done, and not regret it, just act as ordered..
So, when talking about 'weapons of war', we always do have safeties. Basic safety switches for guns, protocols for target acquisition, multiple launch codes from multiple persons to fire a nuclear weapon.
So, greater the potential of the weapon, greater the safety procedure. And a legion of genetically enhanced super soldiers with wargear and weaponry to destroy entire worlds, is, atleast in my books, rather grand weapon.