Still under construction

Hello my name is Marquis Vladimir. This is the first guide I ever wrote and the main reason why I wrote this is because I wanted to know what you guys think about my ideas about how to build Zed the right way. So please leave a comment with input, or call me stupid for choosing a certain item/mastery/rune, but if you do just also explain why ;)

Zed is the first ninja in 200 years to unlock the ancient, forbidden ways. He defied his clan and master, casting off the balance and discipline that had shackled him all his life. Zed now offers power to those who embrace knowledge of the shadows, and slays those who cling to ignorance.

An orphan, Zed was taken in and trained by a great ninja master. Only one other student appeared to be Zed's equal - the master's son, Shen. It seemed Zed could never win the favor of the master, as every match between the rivals ended in a draw. Frustrated and jealous, he sought an advantage. The young ninja ventured into a sealed part of the clan's temple, where he found an ornate, foreboding box. Sensing the dark knowledge within, Zed knew he should not open it, but he peered inside nonetheless. In an instant, shadows touched his mind, revealing techniques that had long been hidden. Now armed with a secret edge, he challenged Shen, and this time he defeated the master's son. He expected praise and recognition in his moment of victory, but somehow the master knew Zed had used forbidden ways, and banished him.

Humiliated, the young ninja wandered for years. His bitterness turned to ambition, and he began to train others in the style of the shadows. As his power grew, so did his circle of followers, but he knew that without the box, his technique would never be perfect. One day, Zed looked at his followers and saw that his students were now an army. He led them back to the temple to claim his prize. At the gates, he was surprised to find the old master waiting, receiving Zed and his disciples as if they were welcome guests. The old man laid his sword at Zed's feet, declaring that he had failed Zed as his master. By banishing his former student, the master had doomed Zed to the shadows, instead of leading him to the balanced path. The old man implored Zed to enter the temple, destroy the box, and lead his followers to balance. The dark ninja followed the master inside. Moments later, the assembled ninjas heard Zed cry out in pain. Mysteriously, he emerged unscathed, and threw the severed head of the master at Shen's feet. Screaming in rage, Zed commanded his followers to slaughter the master's students and seize the box.

That day, the old ninja order fell. Though many students died, some escaped thanks to Shen's heroic efforts. Now the temple is a dark training ground for the Order of the Shadow. Zed rules as the Order's master, and his edict is simple: perfect one's technique, and kill all ninjas who refuse to embrace the shadows.

''Balance is a lie - we are the true ninjas.''
-Zed

Contempt for the Weak [passive]
Whenever Zed autoattacks an enemy below 50% health, he deals an additional 6 / 8 / 10% of their maximum health as magic damage. This effect cannot occur on the same target more than once every 10 seconds.

Razor Shuriken
Active: Zed and his shadows throw their shurikens, each dealing physical damage to the first enemy they pass through and a lesser amount to enemies thereafter.
Damage To First Target: 75 / 110 / 145 / 180 / 215 (100% bonus AD)
Damage to Secondary Targets: 60 / 88 / 116 / 144 / 172 (+80% bonus AD)

Living Shadow: Additional shurikens striking the same enemy deal 50% damage and restore energy.
Shadow Shurikens: 37.5 / 55 / 72.5 / 90 / 107.5 (+50% bonus AD)
Cooldown: 6
Cost: 75 / 70 / 65 / 60 / 55 Energy

Living Shadow
Passive: Zed's bonus attack damage is increased by a percentage. Additional Bonus AD: 5 / 10 / 15 / 20 / 25%
Active: Zed's shadow dashes forward, remaining in place for 4 seconds. Reactivating this ability will cause Zed to switch positions with this shadow. Zed's shadow will mimic his basic abilities. If both strike the same target Zed regains some energy. Energy can only be restored once per mimicked ability. Energy Restored: 20 / 25 / 30 / 35 / 40
Cooldown: 22 / 20.5 / 19 / 17.5 / 16
Cost: 40 / 35 / 30 / 25 / 20 Energy

Shadow Slash
Active: Zed and his shadows create a burst of shadow, dealing physical damage to nearby enemies. Physical Damage: 60 / 95 / 130 / 165 / 200 (+80% bonus AD)
Zed's slash reduces the cooldown of Living Shadow by 1 second for each enemy struck. His shadows' slashes slow enemies for 1.5 seconds. Slow: 20 / 25 / 30 / 35 / 40%

Living Shadow: Overlapping Shadow Slashes striking the same enemy deal no additional damage but will incur an increased slow and restore energy. Overlapping Slow: 30 / 37.5 / 45 / 52.5 / 60%
Cooldown: 3
Cost: 50 Energy

Death Mark
Active: Zed becomes untargetable and dashes to target enemy champion. Upon arrival, he marks the target for death and spawns a living shadow behind the target. This shadow lasts 4 seconds and mimics Zed's abilities. Reactivating Death Mark will cause Zed to switch positions with this shadow. After 3 seconds, the mark will detonate, dealing physical damage equal to 100% of Zed's attack damage plus a percentage of all magic and physical damage dealt to the target by Zed and his shadows while the mark was active.
Bonus Physical Damage: 20 / 35 / 50% of damage dealt
Cooldown: 120 / 100 / 80
Cost: No cost

Maxing out your Razor Shuriken early game gives you a good ranged harras against your enemy. Being relatively squishy early game, you don't want to stand too close too your enemy.
Razor Shuriken also is ideal to last hit creeps from a distance.
I do understand why people want to max out Shadow Slash first because its damage output against multiple creeps is great. But I prefer playing Zed defensive early game, so I don't get CC'd and ganked to dead. Which can happen quite fast, because you are pretty fragile.