Con: If militaries come to believe that having killer robots is critical to national defense (either because their enemies are posing a major threat, or because they're more effective than other strategies or required as a part of an effective strategy) then they will likely oppose this law or refuse to follow it. Even if they manage to resist the temptation to build them as a contingency plan against risks, if they're ever put into a position where there's an immediate threat (for instance: choosing between death and lethal autonomy), they are likely to choose lethal autonomy. It may be impossible to keep them from using these as a weapon in that case, making the ban on lethal autonomy just another ineffectual rule.

If the consequences of breaking a rule are not as grave as the consequences of following it, then the rule isn't likely to be followed.