If God is all powerful, why does he need the Angel of Death? Why did God need the angel of death to carry out the 10th plague to the Egyptians? Couldn't God himself just do it?
A better way to formulate your question is that one passuk (verse) says Hashem will do it (Shemos 12:12 - "And I [Hashem] will pass over Egypt...") and another has an internal contradiction whether the Mashchis/Destroyer will do it or whether Hashem will (12:23 - "And Hashem will pass over to strike Egypt...and He will not give the destroyer permission to enter your houses to strike.").
- The Ramban (to 12:23) explains that the latter passuk merely means that while Hashem is attacking the Egyptians, He will not let the Mashchis, who will be out doing his business elsewhere in the world, to come anywhere close to Mitzraim, where Hashem Himself will be doing the smiting.
- The Chizkuni (to 12:23) explains that the passuk means that Hashem will personally escort the destroyers to Mitzraim, and they will attack the firstborn. "I and not a Malach" (angel) of 12:12 thus means "I and not only a Malach."
- The Da'as Zekeinim (to 12:23) gives this same explanation, though IMO a little more clearly. He, however, seems to take it as a given that Hashem wouldn't go to attack Mitzraim without the Malachim. Not sure why that's so clear.sut
- The Ha'amek Davar (to 12:12) says exactly the opposite explanation: not that the malachim attack and Hashem is there for the show of glory, but that Hashem is attacking and the Malachim are secondary to Hashem to help. He seems to be learning "I and not an angel" as "I primarily and not an angel primarily.
- Shemos Rabbah (17:5) has exactly this doubt: "Some say [the attacking was done] through a Malach, and some say [it was] through Hashem Himself. [According to the latter view,] what does it mean that [the passuk] says "[He will not give permission to the Destroyer to enter your houses] to strike"? ... The Destroyer went and attacked whatever he could find."
That God didn't do something doesn't mean that He couldn't. God is all-powerful; clearly He can do anything He chooses to do. But we see a lesser form of this with powerful-but-not-omnipotent people, too: parents, managers, and leaders all sometimes tell others to do things they could well do themselves. Why? Because they see some benefit in having the other do the task. Along similar lines, then, why could God not see value in having the angel of death, other angels, or people do things that He clearly could do himself?
In fact, a strong case of this is with the mitzvot. Does God need us to do them? We can't do anything that God could not do Himself, and yet He commands us anyway. If it makes sense for God to direct our actions, then surely it makes sense for Him to direct the actions of beings with no independent thought or will.
(This is my own reasoning.)