You have to love the Asgardians. They are always sticking their neck out for Earth. That's why Destroyer was created. He's kind of like the statue of liberty, but instead of being a symbol of freedom, he's a symbol of "don't mess with Earth." You see, the Celestials are pretty much god-like characters, and their plan was for Earth to one day have judgement set upon it. If the Earth failed that judgement, they'd be killed. Odin was like, "hell no," and created Destroyer to fight off the Celestials if Earth got a failing grade. Eventually several people tried to use it to kill Thor, so it didn't work out perfectly, but the intentions were good.
You can't mess with a name like Destroyer. It says it all right there. Protector might be for fitting, but it doesn't have the bite that Destroyer has.