Originally Posted by
Absimiliard
The most logical (and seemingly accurate) explanation for why Zenos didn't get involved with Eorzea sooner is simply that it wasn't his row to hoe. Someone that "done" with life isn't exactly going to go out of his way to intervene on behalf of another ranking Garlean officer without a very good reason, and at that time the WoL was likely beneath his notice.
As for why a man with all that power wouldn't just go do whatever he wanted? Lack of drive. It was made known to us that Zenos's childhood was uh... horrible, to put it bluntly. He was vastly superior to all of his peers, and even the man chosen to teach him the art of the sword could only hold an edge over him due to the Garlean people's inherent deficiency in manipulating aether. Zenos was so determined to master his teacher's skills that he shoved crystals into his flesh, allowing him to utilize their aether. With this, he proceeded to slay his instructor with the man's own technique. It would be the last time he felt much of anything until meeting the WoL, an individual in whom he saw a great rival.
Being raised as a child soldier, or in his case a child supersoldier, kinda robs you of a lot of perspective. There are a great many things in the world you're like as not never going to consider pursuing, and life paths other than those thrust upon you by your caretakers are pretty high on that last. His upbringing aside (noting also that he had caretakers/handlers, as his parents couldn't be arsed to bother with him), his unusual abilities more or less meant he was a pariah. Not just to others, but even in his own mind. He knew he wasn't like them. Prodigious talent has a way of alienating people to an insane degree, unfortunately.