When playing Kha'Zix, your job should be to dive the ad carry and kill him ASAP. once he is dead, it is more important to help defend your carry then to look for more kills. you should be a heavy damage source, and along with smart play styles, you will be equally durable.

*** If you like my build please post comments and give me a thumbs up :D if it becomes even slightly popular, i will add pictures and go into more detail, along with better wording and format.***
please read the whole guide before voting or commenting.

Greater Mark of Attack Damage - more damage? why not?
Greater Glyph of Scaling Magic Resist - late game magic resist. will help you stay alive from that LeBlanc or Annie.
Greater Seal of Armor - Armor to help with jungle minions, and also lets you take less damage from ad carrys.
Greater Quintessence of Attack Damage - once again. damage.

I take 10 in the offensive tree to get the increase of attack speed and damage along with the 10% increase in armor pen.
11 in the defensive tree for the reduced minion damage and damage reflection when hit by minions from Bladed Armor to help with jungleing. also the armor and health bonus will give you more defence in team fights.
i take 9 points in utility for the increase buff duration. Usually, I take Summoner's Insight for the improvement on flash. I also usually take Expanded Mind for the increase in mana over Good Hands because you should not be getting killed so often to where it is worth grabbing.

The items in my build are extremely situational. starting Boots and Health Potions is my personal preference, but Cloth Armor and 5 Health Potions also works just fine. Wriggle's Lantern is not a necessity, but it helps you take less damage from jungle minions and increases clearance time. Getting Mercury's Treads also can be substituted for Ninja Tabi or Berserker's Greaves, depending on your situation. Getting The Brutalizer after can increase your damage significantly, but if you are having a hard time securing kills, or you are getting killed to quickly, you might want to grab a Phage first and go strait into your Trinity Force. If you are doing very well, and looking to snowball, getting The Brutalizer followed by a Sheen will give you incredible damage to give you more kills. Once you have your Trinity Force, your damage will be plentifull. if you are looking for more damage, and you are not having trouble being focused by the enemy team, you may consider going into Bloodthirster or Hexdrinker. If the enemy team is focusing you, and you are having trouble surviving, it will be better you go ahead an grab a Guardian Angel. At this point you have a choice to either go for more burst damage or more survivability. turning your Hexdrinker into a Maw of Malmortius and grabbing a Last Whisper will give you more damage with small amount of durability. If you need more defence from mages, i would recommend a Banshee's Veil. the sheild will help keep you from being stun locked or bursted when diving a carry. Randuin's Omen, Sunfire Aegis, and Frozen Heart are very good choices for defence against ad. Personally i prefer Frozen Heart or Sunfire Aegis over Randuin's Omen due to the cdr Frozen Heart offers and the health/dmg Sunfire Aegis offers.

Other good items:
Force of Nature - move speed, magic resist, and health regen? sounds good; however, the sheild and health from Banshee's Veil is to much to pass up.
Frozen Mallet - the large health increase and damage alone is wonderfull, not to mention the slow,but with your Guardian Angel Maw of Malmortius,and Trinity Force, you should be durable enough to not need the health. still a great choice though.
Warmog's Armor - insane health and health regen. fantastic. you are a assassin, not a tank. unless you are getting an Atma's Impaler, do not get this.
Youmuu's Ghostblade - fantastic item choice. I useually dont get it because I think it is outclassed by the other items on my guide. If you disagree, get it, it's a great item.

Flash - strongly recommended, helps you get in fights and out of fights, combied with your jump, you never die :D
Smite - All jungles take it. you should too.
Ghost - will help you chase down enemys and run away, however, i prefer flash for the ability to jump over ledges.
Exhaust - great choice if you don't take flash or ghost, will help you secure kills and also help prevent counter-jungling. I usually take flash instead for the excape ability, but if you are confident with yourself, give it a shot.
Ignite - adds to your burst damage, and secures kills, however, yet again, flash is still more important
Cleanse - I can understand this choice, but I would not try it especially as a jungle.

Maxing your Q skill first will give u incredible damage early, and will scale well into late game. however it is important to get each of the three available skills by level three to insure jungle clearance and ganking potential. Maxing your W skill second will increase your damage even more, and will also give you survivability through its healing affect.your E skill is very important for excaping from enemies and chasing enemies. This offers great utility, but can wait to be upgraded till later. your ultimate should always be upgraded asap.
Evolving your skills.
I personally evolve my Q skill first for the increase in damage and range. <--- necessity.
the second evolution can go into your w,e, or ultimate skill, depending on how your game is going. If you are being focused to hard as to where you cant even be useful, you might want to consider upgrading your ultimate, however useually upgrading your jump (e skill) will be enough to keep you alive. If you are not having much trouble surviving, upgrading your W skill can greatly increase your damage, and will give you a slight increase in AOE damage. depending on what you upgraded second, your third choice will vary, if you are STILL being focused, having your E or R skill (depending on which one you got second) will insure survivability, while if you are not having trouble surviving, your w will increase your damage greatly.

Overall, my desired evolution order is first Q, second W and finally E.

When jungling, if I am on purple team, I prefer to start wolves and grab the ancient golem buff, followed by a level 2 gank to bottum lane. When I do this, i will upgrade me leap(E) second instead of Void spike(W). Most supports will not have warded yet, and it is almost a certain kill. If i am on blue team i will start wolves followed by ancient golem buff, then wraiths red lizard buff, and mini golems. Depending on how the lanes are going, I gank accordingly.