The reason i chose to build this way was because alot of the time if you get caught with kha zix you die.
With this build and upgrade pattern you have alot of survivability along with the assassination capable damage.
I upgrade the ultimate first because the speed allows you to pursue in the jungle and when you gank, while adding alot of extra damage and cc. This is also a good choice because kha zix takes 50% less damage while stealthed. Some might think its pretty useless because he is invisible anyway, but agaisnt champions that he has a tough time fighting like lee sin who can unstealth the bug this give you a huge advantage. For one when his ult is upgraded, kha is stealthed for 3 seconds and can use the ability 3 times that is a maximum of 9 seconds you you could block half of incoming damage
To up the standards for durability you max void spike first for a greater healing effect
once you hit lvl 11 you are bound to have your warmog already and have probably on average 3-4 kills. Upgrading leap gives you that reset needed to endure huge teamfights where you can go in deal damage and have the option to continue to pursue or jump out because you are low. although this rarely happens, if you get focused hard enough you can still die pretty fast.
Now, a major reason a couple of my friends dislike this build is because they think without the upgraded Q bugman doesnt have enough damage to destroy bigger targets, but since the recent nerf on kha zix his bonus q damage isnt that significant in the early levels, whereas by level 16 6% can be close to 300 damage