Albert Barnes Commentary Numbers 16:38

"even the censers of these sinners against their own lives; and let them be made beaten plates for a covering of the altar: for they offered them before Jehovah; therefore they are holy; and they shall be a sign unto the children of Israel." — Numbers 16:38 (ASV)

These sinners against their own souls — That is, “against their own lives.” By their sin they had brought destruction upon themselves.