لَنُبَيِّتَنَّهُ وَأَهْلَهُ ثُمَّ لَنَقُولَنَّ لِوَلِيِّهِ مَا شَهِدْنَا مَهْلِكَ أَهْلِهِ وَإِنَّا لَصَادِقُونَ
We shall attack him (Salih علیہ السلام) and his family at night, then will say to his heir, We did not witness the destruction of his family and we are really truthful - 27:49.
The plan was that all of them together should attack him and his companions in the darkness of night, and kill them all. And when the claimant of the blood would make the claim, they would say that they did not kill him, and also they did not know who had killed him. In saying so they would have uttered the truth because no one could have known who had killed whom in the darkness of the night.
The point to note here is that all the acts of infidelity, disbelief, killing and pillage were being performed by the infidels, rather than those among them who were hardened criminals. Yet they were careful that they don't speak a lie or get blamed for falsehood. This shows how great the sin of falsehood is, that even the perpetrators of heinous crimes avoid it for the sake of their self-respect.
The other thing worth noting in this verse is that the one whom these infidels had mentioned as the Wall or heir of Sayyidna Salili (علیہ السلام) was a man of his own family. So, why did they leave him out from being assassinated? The answer to this question is that he might have been the successor from the lineage point of view, but otherwise he could be an infidel and be a party with the infidels. There could have been the consideration that he might claim the compensation for the blood of Sayyidna Salih (علیہ السلام) because of his relationship with him. The other possibility is that he might have been a Muslim but enjoyed a prominent position and hence there could have been the risk of uprising and reprisal among the people. Only Allah knows best.