At this, a believer from among the relatives of Fir’on, who had kept his faith hidden, said: “Will you kill a man merely because he says: `My Lord is Allah, specially when he has brought you clear signs from your Lord? If he is lying, may his lie be on his head; but if he is speaking the truth, then some of the dreadful things he is threatening you with may very well fall on you. Surely Allah does not guide the one who is a lying transgressor.
Meaning:-
Transgressors: a person who is actually breaking the law, offend; sin.
dreadful: terrible, frightful, horrible, extremely disagreeable.