1) Those who gave the order (position of power). Often a chain of command.
2) Those who planned it (usually the same or merged with group #1).
3) Those who implement the plan. The engineer, the soldier, the distributor (all down to the line of the logistics driver who delivered the evil TV), etc.
For some strange reason group #1 is called executives. They do not execute much though; they order others to, but execute sounds like they work hard, I guess. Those are indeed the most liable, and we must explain the why. Think Hitler, Himmler, Stalin, Mielke, etc. Lower down the line you get the Einsatzgruppe commanders.
The soldier who executed Jews (among other minority groups, mind you) with a gun is liable, but part of a power structure. If he refuses, he can and will get in trouble. Mass murder generates trauma, so it is executed by pawns or pawns of lieutenants. NKVD solved this by using others to execute.
Ergo: the higher up the chain, the more liable these people were as they had more power to say no.
That someone else would say yes is a red herring excuse. An engineer is in a position of power. They're an educated specialist in their field, they get paid for their job (not forced to implement in West), can say no. They can even blow the whistle. If those in power cannot find a fool to do their errands, they must do it themselves (and they cannot as no expertise), or lay down the idea.
The gas used to gas people in concentration camps didnt magically end up there. They weren't lied to that they'd get a shower by people who didn't knew what would happen. They knew or could've known, but they chose not to act.
(I'm using older examples of genocide as they are established and well documented; I am aware of more recent examples, but 1) this hasn't been solved legally, 2) I leave that comparison as an exercise to the reader.)