There was a discussion on the experiments by Stanley Milgram about obedience to authority, and it applies here.
When a consultant gets hired for advise or to "get stuff done", and the result of this is massive profits with a secondary effect of death and destruction, then the consultant is (i.m.o.) morally responsible for that second part as well. And, it seems, legally as well (but let me guess nobody goes to jail).
The consultant may have been hired only for the first part, "get stuff done", and the pay may have not been much, but that has nothing to do with responsibility. The minute you understand the possible consequences of the things that you contribute to, you become partly responsible for it.
As the Stanley Milgram experiments show, most people will do horrible things to others when they can tell themselves that someone else is responsible. And, I may add, at the McKinsey level, consultants may even get hired to take on the authority role, to excuse financial crimes, or worse.