We know that God allows the devil to kill (John 10:10), yet protects in situations like Job.  God kills in cases like the flood and Sodom.  How can we know if a death is of God or the devil?

The thief mentioned in John 10:10 is not the devil.  Jesus defines the thief as “all who came before me” (v. 8), a reference to the Jewish religious leaders of his day who were leading the people astray in an effort to become rich (v. 1; cf. 9:40-41).  Thus, the thief in John 10:10 would refer to those whom the devil was using for his purposes, but not necessarily to the devil himself.  The killing mentioned in John 10:10 is in contrast to the abundant (eternal) life Jesus was offering, thus making the death resulting from the killing spiritual in nature rather than physical.  The killing done by the thief (the religious leaders of Jesus’ day) was spiritual in nature due to the error towards which they were leading the Jews.

The only instance which comes to mind concerning Satan taking physical life is a situation in which God specifically forbade him to take life (Job 2:6).  We also know that death comes due to sin entering the world (Gen. 2:17).  Oftentimes it is the result of sinful actions of ourselves or others (e.g., Ex. 11-12; 1 Kings 22:17-24).  So it could be said that Satan indirectly had a hand in it, but one’s own culpability should not be ignored.

We do know that God has taken life or allowed life to be taken as punishment for sin (e.g., Gen. 13:6-11; 1 Kings 13:20-26) or as a reward for the righteous (Is. 57:1-2; cf. Lk. 16:25; Ps. 116:15).  Yet the reason we know any of this is because it has been revealed in God’s Word as the specific, implied, or likely reasons for deaths in specific situations.  Without further revelation, of which none will be forthcoming (Jude 3), there is no way for us to know why a particular death has taken place and which supernatural being, if any, is responsible or had a role to play in it behind the scenes (Deut. 29:29).