>>212404456No one is incorruptible. Gandalf wouldn't even touch the Ring because he knew the temptation to try to use its power would be too great and even using it with good intentions would lead to him becoming a tyrant just like Sauron, maybe even worse because it would be done with an air of righteousness.
Saruman fell victim to a mix of his own arrogance and ego, his envy of Gandalf for his ease in befriending and allying with the various peoples of Middle-Earth (Saruman is the guy you go to for advice, Gandalf is the guy you invite for dinner), frustration with the slow passage of time in the form of a human, despair over believing Sauron couldn't be stopped so it was better to just grab what you can, and classic power hunger. He had been chosen to lead the wizards due to his wisdom and knowledge, but great stature doesn't always guarantee great character.
He WAS punished for his betrayal as well, when he "died" his spirit looked to the West, to Valinor, but was denied a return, and scattered by a breeze, his power is permanently broken and he'll remain formless in Middle-Earth for the rest of time, like Sauron after his final defeat.