Yes, i agree that Zeratul isnt viable because of stuns, but look at links down below.I played Zeratul with this same build and we were dominating all game.
This is great against single target and against enemy healers because you can burst them down in second.
At level 16 i took Rending Cleave because i think thats better talent than Berserk,we had took Searing Attacks and thats enough of burst damage for 1 target. Rending Cleave is better at the late game when you can do lot of AOE damage with Cleave.
Look at the photos below, these are from my gameplay and i really love this build, if you want to take Berserk sure why not :D



http://imgur.com/64GsEN6,kft31mp
http://imgur.com/64GsEN6,kft31mp#1

Auto attack Zeratul is not viable at all, because this game gets dominated by CC (stuns).

Even if you would want to play the best auto-attack Zeratul there is, you should get Beserk at lvl 16 instead of Rending Cleave.