Most people when they go Kassadin they go AP since he scales with AP. AD Kassadian is actually viable though your passive is really strong since it gives you loads of atk speed during 1v1s. Also your W is literally free sustain for mana. Your ult is an awesome gap closer to get out and in. So you can basically ult in land 3 auto attacks with your W and you literally do like 1000 damage. Combined with your E & Q AD KASSADIN IS EXTREMELY STRONG.

Pros:

-Extremely effective in 1v1 your passive gives loads of atk speed
-Does Loads of damage in AP while going AD
-Generally not expected to go AD and definitely not ADC
-Strong silence with Q
-Free mana sustain and extra damage boost with W
-Strong Slow with E
-If an enemy builds AP you have Ad to counter and vice versa.Is sorta of like tristana

Cons:

-Extremely squishy early game
-Hard to play without the right team comp/support
-Hard to get Cs early game which can lead to you losing the game

An ADC is almost always a champion who has a 450+ range for auto attacks. Whereas, Kassadin has only 150. SO LITERALLY HOW CAN YOU WIN by going ADC KASSADIN?

You have to have a really good support and team comp for ADC KASSADIN to work. Any support that has a heal like soraka is OP with Kassadin since when your farming they can heal you to negate most of the damage you take.They can also harras the ADC making them not harras you and losing farm.

ADC KASSADIN CAN WORK BUT YOU SHOULD GO TOP LANE OR MID IF YOU DONT HAVE A GOOD SUPPORT. I RECOMMEND ONLY GOING ADC KASSADIN WHEN YOU HAVE A FRIEND WHO KNOWS HOW TO SUPPORT AND KNOWS HOW ADC KASSADIN WORKS.

AD KASSADIN IS EXTREMELY OP IN 1v1s. He can literally silence you and once he pops his W and if he has a full build your going to die in like a second and if you try to run he can just ult to kill you. This is why you should go ADC KASSADIN since despite the fact you're going to get poked like crazy and you might actually feed 3 kills early game and even lose bot turret you will carry like hell late game. Basically if you go ADC KASSADIN vs. a Jinx or Ezreal you can literally dodge their ult with your own and your AD may be less than theirs but combined with the W you literally do 100 magic damage plus 300 AD from your auto attacks.Once you get your ult you can wreck the lane. So just play passively till you reach level 6.

In patch 4.4 Kassadin has been hit pretty hard by the nerfs. His W, and passive have hurt going AD Kassadin a bit. Before his passive used to give atk speed which was the main part of going AD Kassadin this has been replaced with a nice ability to move through units which is pretty good but now we will have to build Atk speed items to compensate for this. His W which used to give mana on hit and the active which gave loads of burst damage is now nerfed a bit as well. His W now gives a neat 20 magic damage per basic attack and an active which restores mana and does an additional amount of magic damage for the next basic attack. This nerf isnt as bad as it seems for AD Kassadin because if you go AD on Kassadin you don't really need to use your abilities a lot meaning that you will still be able to sustain yourself with the mana that the W gives now. The passive on the W now gives some more damage and can help you cs a bit which is pretty helpful in early game. Also the fact that the W now requires no mana to activate makes Kassadin have more sustain than ever if u go AD. As of Kassadin's other abilities they're not as impacted by the nerfs.

Now that Kassadin really has no atk speed you're going to want to build an item that has atk speed. I recommend building Nashor's Tooth or Zephyr or even both. Zephyr has a nice atk speed bonus (+50%) whereas Nashor's Tooth has 50% as well but has 20% CDR compared to Zephyr's 10% CDR. Zephyr's tenacity is very nice and I recommend building this if they have lots of CC but generally Nashor's Tooth is better for its higher CDR and the passive which gives extra damage towards your basic attack. To build these items now you will have to take some items out of the original build i prefer taking out the Black Cleaver since the black cleaver isn't needed since now you're going to be even more burst orientated with the patch so you don't need that armor pen. Though Black Cleaver is good Zephyr for instance will give you the 10% CDR plus more atk speed and some movement speed.

AD KASSadinN IS NOW BETTER THAN EVER the scaling has increased with your W. Which means more damage even if you don't build ap. Not only that you now have more movement speed which helps if your going AD Kassadin because you can run away faster. Also you now recieve more mana which is always nice. The impact isn't that great but makes AD Kassadin even better.