During the War of the Ancients, thousands and thousands of years ago, Maiev Shadowsong was in roughly the same age group as the substantially better known 'heroes' like Tyrande, Illidan and Malfurion. She wasn't always the walking pile of steel and pointed edges that she is today. She was originally a Priestess of Elune, right alongside Tyrande, and one aspiring for greatness at that, aiming to become the High Priestess of Elune one day.
Unfortunately for Maiev, her aspirations were all for naught.
Dejahna, the High Priestess of Elune at the time, overlooked Maiev in favor of Tyrande in her dying moments after a catastrophic battle with one of the Legion's lieutenants. The decision was largely not based on merit, but rather the fact that Tyrande had been favored by Elune seemingly arbitrarily. Naturally, Maiev was not happy with this decision, and wasn't exactly quiet about it... but knew better than to speak out too loudly about it, to prevent causing unrest in an already war torn people. Trying to decry the new High Priestess in the midst of a battle for your very existence tends to not be particularly helpful. Maiev sucked it up, and continued fighting the good fight.
That sort of thing was not particularly new to Maiev, either. She tended to have very strong opinions about the leadership of the resistance, both for and against, but understood that the Night Elves (or whatever you want to call what they were at the time) needed to have a united front. A respected Sister of Elune calling out Desdel Stareye, as incompetent, short-sighted and racist wouldn't have been helpful at the time either, no matter how true it may have been. Maiev restrained herself from dividing the resistance by calling Desdel a heel, but she certainly knew what she was talking about. The resistance nearly fell apart under Desdel, undoing all that Kur'talos Ravencrest, the previous leader, accomplished before his assassination.
Luckily, Desdel wasn't in charge very long. His incompetence caused him to meet a very premature end at the hands of the Burning Legion, which caused Jarod Shadowsong, Maiev's younger brother, to assume command of the resistance. Jarod was able to pull the resistance out of the slump Stareye put them in, and the War was eventually won.
After the war had come to its conclusion, Tyrande chose Maiev as her second-in-command, but that position didn't stick. After Illidan created a new Well of Eternity and attempted to murder much of the Elves' leadership via arcane holocaust, Malfurion Stormrage and Tyrande decided Illidan couldn't be trusted anymore and locked him up indefinitely. This is the point where Maiev becomes what we know her as today, the Warden. Appalled by Illidan's reckless use of arcane magic, Maiev left her position as Tyrande's Second in Command and became Illidan's jailor, to ensure that he wouldn't threaten their people or world ever again.
At Malfurion's request, Maiev Shadowsong formed a group of elite warriors known as the Watchers. It wasn't only Illidan that they would be jailors for, but all of the Elves' most dangerous criminals. Consider the Barrow Deeps their equivalent to a place like Alcatraz. The highest security prison around, housing the worst of the worst. It was Maiev's job to keep them there.
It is in the Barrow Deeps that Maiev stood vigilant for over 10,000 years, in the interest of protecting her people. When she wasn't hunting down criminals, she lived in the Deeps with the Watchers, who had essentially become her family over all of that time. Naisha, for example, was not only Maiev's lieutenant, but something of a sister... or something else, if you get a kick out of that sort of thing.
After those ten thousand years, all of Maiev's work and effort was dashed in one day. Tyrande Whisperwind, the chosen leader of her people and one of the individuals that locked up Illidan and assigned the Watchers to keep him locked up... stormed the Barrow Dens, murdered the Watchers (including Califax, a Keeper of the Grove) in cold blood, and freed the Betrayer.
Think about that, dear readers. Really think about that. Maiev gave up everything to keep the Elves safe... and the Elves thanked her by slaughtering her closest friends en masse and releasing all of those criminals she was meant to watch over. It wasn't just a rogue group of Elves, either. It was Tyrande freakin' Whisperwind, the chosen of Elune, that committed those murders.
Maiev was, of course, not pleased with this, and that's the turning point where more and more of the Warden's personality becomes replaced with pure vengeance. She was betrayed in every sense of the word, and Illidan still belonged locked up no matter what insanity Tyrande pulled. Maiev would make sure that he got thrown in prison again. So she, and the remaining Watchers, gave chase.
Their chase eventually brought them to the Tomb of Sargeras, where Illidan sought out the Eye of Sargeras. Unfortunately for Maiev, he'd already found it when she arrived, and intended to bring the tomb down on top of her. Maiev managed to escape, but Naisha, Maiev's second-in-command and closest friend, did not.
Maiev grieved, and lost even more of herself to the grip of vengeance. It's interesting to note that this wasn't only represented in Maiev's actions and dialogue, but it was built right into the nature of her class in Warcraft III. As these events happened, you leveled up and became a much more dark and brooding person, until you finally achieved the ability Avatar of Vengeance. The ability can be described as revenge given a physical form, and I don't think anything displays Maiev's evolution as a character better than that.
After a brief reunion with Malfurion and Tyrande (much to her distaste), and meeting Kael'thas who had allied with them briefly, she realized that neither Tyrande or Malfurion had any real interest in tracking down Illidan and bringing him to justice. It was at this point that Maiev figured out that she was pretty much in this thing on her own. Tyrande had lost her mind, and putting faith in Malfurion's judgment was certainly out of the question because of his ties to the High Priestess. If Tyrande isn't going to listen to reason, her beau isn't going to, either. Maiev, quite simply, realized that the world had gone mad around her. She watched over a criminal for 10,000 years, and woke up one day to a world that not only freed criminals, but embraced mass murderers as saviors.
The was reinforced when she finally tracked Illidan down... and found that Malfurion got there first. And not only did Malfurion get there first, he let Illidan go free again.
Maiev continued her chase, naturally, growing more and more disenfranchised with her own people, potentially to the point that chasing Illidan was no longer a matter of protection and more a matter of pride and, again, vengeance. Her need to bring Illidan to justice drove her to chase him all the way to Draenor.