Saruman wasn’t
evil when he arrived in
Middle-earth: the
Valar would never have sent him if he were. He was
proud, however, though not nearly so much at first as later.
In
Unfinished Tales, there is a section called “The
Istari”. It tells that the
Vala Aulë chose
Curunír (Saruman) to oppose
Sauron, who was also one of Aulë’s people in origin.
Yavanna, Aulë’s wife and herself a powerful Vala, insisted that one of her
Maia accompany Curunír, and she chose
Aiwendil (
Radagast). Though Yavanna was not evil in any way, sometimes her actions seemed driven by competition with Aulë. (For instance, when Aulë made the
Dwarves, Yavanna insisted that she be allowed to make
Ents to look after
trees.) Curunír was the leader of the Istari, or
Wizards (wise ones), who were sent to oppose Sauron by strengthening
Elves and
Men, but were forbidden to rule over them, or to oppose Sauron through their own force. (Basically, they were to offset his evil influence in Middle-earth: separating Elves from Men, and forcing
creatures to do his wicked will.)
Curunír arrived first, around 1000, and it seems Aiwendil was with him. Aiwendil’s heart seems never to have been in the mission, though: he preferred
animals and especially
birds, to Elves and Men, and became a friend to beasts and birds: as
Beorn put it, “not a bad fellow as Wizards go.” Curunír resented Aiwendil: he had an weighty, important task at hand, and Aiwendil – Radagast – just wasn’t up to the job, in his opinion.
Olórin –
Gandalf – was also sent, but he was chosen by
Manwë as “the third” (after Curunír and Aiwendil) because “Olórin was a lover of the Eldar that remained.” Olórin objected “that he was too weak for such a task, and that he feared Sauron. Then Manwë said that that was all the more reason why he should go”, and appointed him third. But when Manwë’s wife
Varda (
Elbereth Gilthoniel, the Star-kindler) heard Manwë, she looked up and said, “‘Not as the third;’ and [Curunír] remembered it.”
When Gandalf arrived last in
Mithlond,
Círdan the Shipwright, who may have been the oldest surviving
Elf in Middle-earth at that point, and who was extremely wise and far-sighted, had
Narya,
the Ring of Fire, in his possession. He gave it to
Mithrandir (Olórin-Gandalf). Saruman eventually discovered this, “and begrudged it, and it was the beginning of the hidden ill-will that he bore to” Gandalf.
Silmarillion says (bolding and square brackets mine),
…Mithrandir … doubted the darkness in
Mirkwood, for though many deemed that it was … the
Ringwraiths, he feared that it was … Sauron …; and he went to
Dol Guldur, and [Sauron] fled… . [Then] was [the] first …
White Council, and therein were
Elrond and
Galadriel and Círdan, and other lords of the Eldar, and with them were Mithrandir and Curunír. And Curunír (…Saruman …) was chosen to be their chief,
for he had most studied the devices of Sauron of old. Galadriel … had wished that Mithrandir should be the Lead of the Council, and Saruman begrudged them that, for his pride and desire of mastery was grown great; but Mithrandir refused the office, since he would have no ties and no allegiance, save to those who sent him [i.e., the Valar], and he would abide in no place nor be subject to any summons. But
Saruman now began to study the lore of the Rings of Power, their making and their history.
In the “
Council of Elrond” in
FotR Elrond indicates that in “study[ing] the lore of the Rings of Power, their making and their history,” Saruman was beguiled by the power of the One Ring, and thus he fell from grace.
It is perilous to study too deeply the arts of the Enemy, for good or for ill.
The White Council was formed in 2463,
about the same time that Déagol, Sméagol’s friend, found the One Ring in Gladden Field: Sméagol murdered him and took the Ring.
Isengard was never a
fortress of
Rohan. It was always an outpost of
Gondor; but the house of the hereditary lords of
Angrenost failed, and the
Dunlendings took control of it in 2710, according to the Tale of Years.
In 2758-9, Saruman and Gandalf parted ways in how they responded to the crises in
Arnor and Gondor. The Long Winter struck
the Shire, and Gandalf came to the aid of the
Hobbits and other folk living in the ruined kingdom; while Saruman took possession of Isengard, at least nominally as its steward for
Beren, the 19th
Ruling Steward of Gondor. One of his purposes might have been to use the
palantír of
Orthanc to look for the Ring. Yet even then, he probably wasn’t yet fallen. When
Théoden asked Gandalf about Saruman in
TT, Gandalf said,
How far back his treachery goes, who can guess? … He was not always evil. Once I do not doubt that he was the friend of Rohan…
Saruman began to experiment with making Rings. He knew where the One Ring was lost, and began searching the Gladden
Fields for it around 2851 (according to the Tale of Years). Sauron had been searching there much longer: that’s probably why he chose Dol Guldur as a base of operations. Gandalf saw
Thorin Oakenshield’s father, Thráin son of
Thrór, in Dol Guldur that year, discovered that Sauron was there, and urged the White Council to drive him out; but Saruman overruled him because he hoped Sauron’s presence would make the Ring easier to find.
But in 2939, according to Tale of Years, “Saruman discovers that Sauron's servants are searching the
Anduin near Gladden Fields, and that Sauron therefore has learned of
Isildur's end. He is alarmed, but says nothing to the Council.” Two years later, Gandalf meets Thorin Oakenshield on the road to
Bree, and they concoct a crazy scheme to hire a
Hobbit as a
burglar. And during that adventure, Gandalf left them at the entrance to Mirkwood so that he could attend the White Council and help drive Sauron out of Dol Guldur. (Insuring, incidentally, that Sauron was too distracted to bother Thorin & Co in Mirkwood, or send any assistance to
Smaug.) Saruman agreed to this because neither he nor anyone else knew
Gollum had the One Ring, and he was afraid Sauron might get it first. And the One Ring – changed hands. And no one knew that, either.
It isn’t clear exactly when Saruman fell, as Gandalf pointed out. From the beginning he was proud, and looked down upon Radagast (partly because the “dopy” Radagast was thrust upon him by his boss, Aulë: Saruman didn’t want him, and Radagast probably didn’t want to go); eventually, he despised Radagast. When Varda said Gandalf wasn’t “the third [Wizard],” Saruman wondered what she meant, and it bothered him. He resented Círdan’s gift of the Ring of Fire to Gandalf. He begrudged Galadriel’s promoting Gandalf to head the White Council.
But most of all,
Saruman studied Sauron and Sauron’s black arts in great detail. Like Sauron, he was one of Aulë’s folk, and though Sauron was stronger, Saruman was powerful, too. And somewhere along the way, as
Treebeard told
Merry and
Pippin, Saruman began “
plotting to become a Power.” To do that, he needed the One Ring. And he fell.
My bet is that he made that fateful decision sometime after he entered Isengard in 2759 but before 2851, when he overruled Gandalf at the White Council in order to leave Sauron alone in hopes of finding the One Ring for himself. Gandalf chose one way; Saruman chose the other; and Radagast wandered off.