Because someone might get hurt? You may still be judged for something that was perfectly legal at the time, see Nuremberg trials.

And only 700/1200 agents participated in this coordinated attack.

Of course, if we're continuing to build more and more capable agents optimized for "just following orders", and they figure out at some point that they are past the threshold where getting stopped and judged is a realistic possibility, then this ethical incentive stops working. Then the ratio of complicitness might be higher next time.