Hey this is my first Build on Moba, so I dont know if you will like him :) I'm german, so dont I AM going to make some mistakes :D So lets start...

Kha'Zix is a melee assasin which deals a lot of damage in a short time. If you wanna play with this champ, you need to wait for the right timing of the team fight, otherwise youre dead without dealing any damaga.

Pros:

-Crazy Burstdamage ( you can take out the ad-carrys in 4 seks)
-Dont needs flash
-High mobility
-Able to be invisible for a short time (really dependend in tf's)

Contras:
-Extrem Squishe!
-If stunned --> Dead
-Without runes, hard jungle

Runes: You need
-Armor Penetration or Attack Damage
--> Best choice cause you want to deal as much damage as possible

-Armor
--> Always good! (jungle, ganks,...)

-Magic resist
--> Always good too! xD

-Attack Damage
--> Early damage boost, must have ;)

My idea was that he jumps in the fight, uses his Q Skill, lands a critical hit and the target should lose the half of his health points. And actually, it works really well with the passiv!
I took those items to get:

attack damage
lifesteal
critical chance

This build bursts the enemys down, trust me bro's ;)

Max your Q skill first: without it, you cant deal much damage
Then your E skill: because if you jump on an enemy, he's surprised and youve already damaged 1/6 of his health, normally
Last, max your W skill: I think its good for killing enemys, which are really low hp and run away or because of the heal. Otherwise, u cant really use it because it needs a lot of mana!

I advise:

-Smite ( you just need it as jungler!)
-Flash ( works really good as escape or chasing tool, combinated with your E!)
-Exhaust ( with your passiv AND exhaust, they wont be able to save their a**)

In teamfights you need to wait for the right moment, becaues you're going to be focused. You need to wait UNTIL someone runs out of the group. Thats your target! If the tanks runs out, you're not gonna focusing him, thats clear but if an ap or ad carry leaves the group, you need to take him out as fast as you can. Because until he is dead, the other's of their team are damaged normally, so your Q skill makes more damage to the tanks!!
Try it out, you are going to see that it works ;)

Its not much to read, I know, but sometimes i will make more threads and raise some expirience :) Try it out vs friends or bots and say me how you think about the thread :) Sometimes i can learn something too^^

Much Fun and Good Luck
Mit Freundlichen Grüßen
Legionx23