Item choices
i absolutely hate getting 
on Kha because i feel like i'm just delaying more important items and spike far worse. And if you get 
, dont upgrade it if it's not the last item slot left. It's just not worth it on Kha.
I mostly go 
into 
, if the enemy team is packed with ap dmg i've bought 
sometimes. The following items on Kha can be pretty much everything and it heavily depends on the situation.
Enemy team has some ap dmg that may be dangerous for you?
You just wantmore reliable assasinations? 
Enemy team packed with cc making it difficult for you to execute ganks properly or kill targets? 
The teams are already grouped and the enemy team is focusing you super hard? 
One enemy that can kill you almost instantly and you are the highest priority for this enemy to kill? 
or even 
can work on Kha.
You need the dmg to kill tanks aswell because you have to basically solocarry your team damagewise? 
/ 
I often end up with a build like > 
> > 
/
/
> > >
In general i'd like to say about items that almost everything that gives ad without crit is a good choice in some situations. I really like getting some bruiserlike items that help surviving like the ones listed above since i never felt that i lack any dmg.
Skills
Always max Q>W>E, always evolve Q first, always evolve E second (i've never seen a situation where its not better to evolve E second)
First clear: if you want to gank early go Q|W|E, if you want to fullclear fast and healthy, go Q|W|W|E and then continue to max Q (better sustain and aoe dmg while clearing if you put 2 points in W)
For the last evolved ability it depends. Most of the time i end up evolving my R just because it gives me more defensive options in teamfights. If your team comp is poke reliant or you want to have an answer to enemy poke, evolving W might be better.
Playstyle
Early game: You are strong in 1v1 situations, especially when you hit lvl6, your early ganks are pretty good and allow you to snowball if you get 1 or 2 kills before hitting minute 10, but as always i recommend only going for safe ganks since you're not that strong in a countergank situation (it's harder to get your damage off dealing with 2 opponents) and don't want to waste time. In a gank, never blow your E before you absolutely have to, wait for the enemy to burn his escape/flash and then follow up with E.
Your early clear speed is good and becomes really good when hitting 6. Look for oppurtunities to counterjungle or catching the enemy jungler offguard when he is running around with half hp or sth.
Champs you should definitely avoid early are shaco, lee, nidalee and elise (at least these are the ones coming to my mind right now; shaco and elise aren't easy to isolate, nida and lee can just straight up kill you in a 1v1 and all those 4 can chase you really good)
Mid game: I really like the midgame powerspike when you evolve your E ability because you instantly become a really strong teamfighter. In teamfights, take your time, only engage when you are sure you can kill someone. Watch out for enemy carries to be isolated and delete them. Even if you fall behind in the early game, Khazix is always a threat for squishies. If you fail your assasination try to buy time with your ultimate and reposition (mostly if you failed an assasination, the target won't be isolated anymore when your cds are up again). I've yet to see a teamfight where i never had a good chance to engage on one of the carries.
Besides teamfighting, if you are not too behind, you can counterjungle a lot and try to catch enemies in their jungle/river/splitpushing
Late game: Pretty much the same as midgame, just be more careful while looking for 1v1s.
is a good last item in some cases
are good boots because 40% cdr Kha (which isn't too hard to get) is OP in my opinion. If you plan on getting 
