Slander is a double-edged weapon: you can hurt the reputation of another but it diminishes your own reputation that you debase yourself so much.
{s} having two blades or cutting edges (of a sword, etc.); acting both ways, having a dual purpose or effect; capable of being interpreted two ways
If you say that a comment is double-edged, you mean that it has two meanings, so that you are not sure whether the person who said it is being critical or is giving praise. Even his praise is double-edged
capable of being interpreted in two usually contradictory ways; "double-edged praise
If you say that something is double-edged, you mean that its positive effects are balanced by its negative effects, or that its negative effects are greater. But tourism is double-edged, boosting the economy but damaging the environment a double-edged sword: see sword
capable of being interpreted in two usually contradictory ways; "double-edged praise"