‘ And Joshua said, “Why have you troubled us? YHWH will trouble you this day.” And all Israel stoned him with stones. And they burned them with fire and stoned them with stones.'
Joshua's declaration was not vindictive. It was a public declaration of the reason for what was being done, a judicial statement of his sentence. Achan was receiving what he had done to others, an eye for an eye. He had brought down great trouble. He must receive great trouble. All Israel participated in the carrying out of the sentence, although not literally. But those who hurled the stones acted on behalf of all.
Achan's execution is mentioned first as being that of the main culprit, then the method of dealing with the remainder. The last part of the sentence is very summarised and we are not told what applied to what. The robe, the gold and the silver would be burned, after which the gold and silver may have been placed in the treasury. The livestock were slain first, and then burned. The other guilty parties would be stoned and then burned. The burning was necessary because all was ‘devoted' and had to be purified in fire (compare Numbers 31:22-23; Deuteronomy 13:16).
The sentence may seem harsh to us. It would not have done to Achan. There are eventful times in history when response to something like this has to be severe for the sake of the future. Those who have the privilege to live at times when God comes very close and acts very openly and vividly, thereby live in times of greater responsibility. We can compare Korah, Dathan and Abiram (Numbers 16) and Ananias and Sapphira (Acts 5:1-6).