The Hydra was so powerful throughout the game. It made clearing waves, clearing camps, killing champions so much easier because you can combo its active so well with your auto attacks and other abilities.
The Dorans Blade... I hate that item, honestly. Riot practically killed it in my opinion with the nerf, but it seemed to be a much better choice over the Machete. I didn't really need any of the items you can build with the Machete. In fact I would actually just sell them eventually, or not even build them at all. This lead to me trying out the Blade. And it worked well with what I wanted.
What I was trying to do, was find a build that allowed me to Carry harder and sooner than anyone else on my team. And, up to now, that's exactly what this build does : D

This is pretty basic.
Get 23 in offence as shown so that you can output as much damage on target champion as possible.
I have the 7 points in Defense just to give me the survivability I need that Kha'Zix tends to lack.
I got the Health Regen because he is so crazy weak until his 6, that he needs as much Health Regen as he can get. The Doran's blade cuts away from the extra Damage he could be doing to the creeps, but it gives him a little bit of health, some "Life Steal" and a little bit of damage, which isn't a bad thing at all.
Anyway, I also took the increased MR and AR because you should have some when banking lanes, otherwise you may have a chance of being eaten alive before you can give it a fighting chance. That little bit of MR or AR could save you from that one hit that could have killed you.
Also take the Vet. Scars for that extra health, which is quite nice to have.
You don't need anything really in utility, unless you wanted the 3 points in Movement or Mana Regen, which is understandable, but you shouldn't really need the MS because of your ulti, and you don't need the mana regen that badly, because unless your Mid is crazy mana hungry, you should be able to pick up a couple blues to sustain your mana for the earlier stages of the game.

The Hydra was so powerful throughout the game. It made clearing waves, clearing camps, killing champions so much easier because you can combo its active so well with your auto attacks and other abilities.
The Dorans Blade... I hate that item, honestly. They (Riot) practically killed it in my opinion with the nerf, but it seemed to be a much better choice over the Machete. I didn't really need any of the items you can build with the Machete. In fact I would actually just sell them eventually, or not even build them at all. This lead to me trying out the Blade. And it worked well with what I wanted.
What I was trying to do, was find a build that allowed me to Carry harder and sooner than anyone else on my team. And, up to now, that's exactly what this build does!

Pros
Crazy Damage
Good Combo Potential
Great Mid Game
Cons
Underpowered in the early game
Weak kit until Ultimate
Item Dependant

Your Q is a MUST for Level 1.
You should try to max it first as FAST as you can! It's the main damage dealer you have and can be combed extremely well to deal Maximum Damage along with you autos and the Hydra's Passive. (Auto-Q-Auto-Hydra Active over and over and over)
Your W is the next priority as it helps slow champions that you gank or it can be used to secure the kill that you're just out of reach to secure.
Your E, the Leap, shouldn't ever be maxed early.
I don't know why anyone would honestly, due to it's long Cool Down and lack of damage. It should be used primarily to initiate a gank or fight, or to close in on or secure kills.

EVOLUTIONS;

First:
Evolve your Q. It gives you a significant bit of extra range and damage which makes successful ganks easier.

Second:
Evolve your E. It is SO important to evolve your E right after Q. It gives you a much bigger Leap range to either jump on fleeing champions, or get away from that team fight gone wrong much easier.

Third:
Unless 'The Hunt Is On' you should figure out which you need more. The extra Ult cast for the speed and Invisibility, or do you need the extra slow from your W? This fink evolution is entirely situational, but I would recommend your W because of the crazy slow, and range of the shots it gives.

You should almost always take Smite. You have no reason not to.
Kha'Zix is way too weak early on to not have Smite.
Flash is also a must have. His Leap (E) is too long of a cooldown, so if you use it and instantly regret going in, you'd need a good flash to get you to safety.
You can also run Ignite instead of Flash if you wish, but I would only recommend that to the more confident players.