“ And it shall be that he who is taken with the devoted thing shall be burnt with fire, he and all that he has, because he has transgressed the covenant of YHWH, and because he has wrought folly in Israel.”
Anything devoted had to be burnt with fire. By taking the devoted thing the culprit had made himself and all that he had part of ‘that which was devoted'. Thus all must be burnt with fire to remove contamination from Israel, and to remove the devoted thing from the camp of Israel. Sadly that may have included not only all his possessions but also his close blood relations (Joshua 7:24). They would share his tent and it is doubtful whether he could have dug a hole and hidden what he did in the tent without them knowing. They would therefore be seen as guilty through complicity.
Note the two charges. He had broken the covenant of YHWH and he had wrought folly in Israel. It was wrong both towards God and towards man, both religiously and morally. ‘Wrought folly in Israel' was a standard phrase for a heinous and grievous wrong (Genesis 34:7; Deuteronomy 22:21; Judges 20:10).
While we do not have to defend the actions of God, especially in such a pivotal and vital situation as this, it should be noted that ‘all that he has' was open to interpretation. Joshua and Israel interpreted it to include all blood relations because that would be the interpretation put on it by the custom of the times, and because they would be seen as guilty of complicity in the crime, but that is not strictly what YHWH said. In these cases God's purpose is often expanded on by man's own ideas. However we must recognise that by his action Achan had allied himself with Jericho, and thus condemned his blood relations just as Rahab had aligned herself with YHWH, thus saving not only herself but also her blood relations. It is interesting that his wife or wives were not said to be included, although it may be she was already dead.