I like this topic, brother! Thanks for sharing it.
Me, I do believe in gardian angels. Though often people tend to idolize them (which is even warned about in Colossians 2:18!) by often thinking angels are beings like God who have their own agenda and act on their own, and they leave God out of the equation. Angels . . . are messengers. They are under command of God.
Now, I believe in Guardian Angels, because of how in the Word of God they do mention how they minister to us and He will charge His angels over us to protect us (in the verse you mentioned, Psalm 91:11).
But also, there are countless times I should have died when I did not, and know for certain that it was because God held firm to His Word in Psalm 91:11-12.
Yet still, in one situation I saw one when I was a kid. My sister and I were sleeping in the living room of the apartment we were in (we were kids at the time). She was on one couch and I was on another. Well, I believe it was close to 1 o'clock in the morning and I woke up. Not sure why I woke up, but I was facing my sister on the other couch, and I saw a tall man standing over her. I called out my sister's name to make sure it wasn't her (just in case I were seeing things

) and I noticed she was definitely still sleeping.
But, the man was tall and had one of his legs over her (one on the ground, the other on the couch over her) as in a guard or watch position. He turned and looked at me for a moment, and then went back to watching over my sister. Recalling it all, He didn't have this white robe that everyone envisions angels in. I could tell he had some sort of "lightweight" armour on (best I could describe it) that didn't look protective, but I knew it was very protective. I also, didn't see a sword on his side, or any weapon, but I
knew that if he needed one it would be instantly at his side. Just something I could tell, like with his clothing being armour though it didn't look like it.
After seeing all this though, I closed my eyes and went back to sleep.

That is a main way I knew it was an angel, because of the peace and the lack of fear in it all. Being a kid, if you saw some strange, big, warrior-like man standing in the dark in your living room over your sister, normally you'd be terrified. But I felt peace and was able to go back to sleep, knowing it was an angel. The times I've seen demons . . . whew, those put a fear in you and a vile feeling that you just wanted it GONE immediately! So it definitely wasn't that either!
So . . .yes, I definitely believe in guardian angels, but always remember that they are under charge from God and not divine beings acting alone and deserving of worship and such. They are here to help us, not bring our eyes off God.
EDITED to add:
Oh, as for the question you posed, which is a good one! I don't know, the way I can see things, is that those who are in Christ belong to God and therefore the devil, just as with Job, must get permission to mess with us and bring harm and troubles to us. However, those lost souls in the world, though God loves them, they are still the children of the devil (John 8:44) until they come to Christ, and the devil has a good deal of rights to do with them what he wants to do unless God specifically forbids him. So, in a way, when such tragedies strike, it is either because God has allowed the thing to happen to His Child for growth, or ultimately to bring him/her home . . . or they happen because the people in the situations are truly under the devil's hand because they have not turned to Christ (And God makes sure they've opportunity in their lives to hear the Truth, so none are without excuse). I don't know though, I guess it's difficult to explain what I mean, without sounding like I give the devil too much, and limit God.