All three signs that God gave Moses demonstrated God's power over life and death, particularly three levels of life on earth:

1) Staff and serpent = plants and animals
2) Leprous hand = life and death of men
3) Nile into blood = nations

YHWH causes death and brings the dead to life.

In Exodus 4:9, Moses poured out the Nile onto the ground like the blood of a slaughtered sacrificial animal: a potent symbol to the Hebrews of God's power over Egypt.