Original sentence: Pretorius was at once chosen commandant-general and speedily collected a force to avenge the massacre of Piet Retief and his party, who had been  killed by the Zulu king Dingane the previous February
Edited sentence: Pretorius was at once chosen commandant-general and speedily collected a force to avenge the massacre of Piet Retief and his party, who had been treacherously killed by the Zulu king Dingane the previous February