I think he can be trusted. Then again, Dumbledore has been wrong before. He was wrong about not telling Harry. He was wrong about not trusting Sirius (to be the Potter's secret keeper.)

OMG I JUST THOUHG OF SOMETHING!

Remember when Hagrid said he was "Keeper of Keys" I always figure it was because he was games keeper. But what if he if one of Hogwart's secret Keepers? That's why first years ride the boat to see Hogwarts because Hagrid needs to show it to them.

back on subject...

Dumbledore, even though he is great, he is still human. His trust maybe miss laid. However, Snape has gone out of his way to protect Harry. The brrom stick year one, alerting everyone year five. He's not very nice, but that doesn't make him a Dark Wizard.