When two persons indulge in hurling (abuses) upon one another, it would be the first one who would be the sinner so long as the oppressed does not transgress the limits.  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ الْمُسْتَبَّانِ مَا قَالاَ فَعَلَى الْبَادِئِ مَا لَمْ يَعْتَدِ الْمَظْلُومُ