Kha'zix passive "unseen threat" makes him deal extra damage to champions, monsters and minions, that are isolated from the others, and this is were kha'zix shines, this passive is amazing to punish positioning, this enable you one shot enemys that walk too far from theyr team on fights, solo drakes with ease and punish random players that overextend on theyr own lanes with few minions.

For kha zix, if you know that the enemy jungler is going to path in the oposite direction, the perfect pathing woud be: Red -> Crugs -> chikens -> wolfs -> blue -> scuttle crab and then you chose between gank or gromp(if any of the lanes is not gankable).

If you know that you can kill the enemy jungler and want to contest a scuttle crab, you can do: Red ->crugs (or chikens) -> wolfs -> blue -> fight for scuttle , this option is good if you want to get an early kill + steal some enemy camps.

As a kha zix, you want to try to gank lanes with few enemy minions, or if you laner have very good AOE (ryze, TF, Oriana etc).
Wen you ganking, you objectie is to hold your E and try to walk up with your R + smite on the enemy, clearing the minions with your W so it isolate the target and use your E to finish or force a flash. Without the isolation damage (kha'zix passive) you do little to no damage.