There are too many flaws in Etzel's theory to mention, but I'll start with the most obvious.

1. Etzel does believe it was Graendal, except that she used Slayer. Therefore he uses Graendal evidence to build his Slayer theory. Erroneous!

2. Slayer can kill anyone with time, true. He has been hired to kill Rand, the most powerful man in the world, and the most dangerous. However, his tactics are very different. He sneaks up when Rand is asleep and stabs him dozens of times with a poisoned dagger. His tactic would accomodate the level of danger he faces, and all the Forsaken would consider Asmodean to be as dangerous as them, as would Slayer. He would thus not openly attack one of the Forsaken who could channel in a second. The gamble would be enormous, and Slayer has shown that he doesn't work this way.

3. The Forsaken would not hire him to kill one of their own. There is no chance that they consider Slayer to be their equal, even if they're wrong. Despite the fact that they despise one another, they disdain all 3rd Age channelers (even Rand), and don't even consider non-channelers. Even with his gifts, none of them truly consider him to be dangerous to the point of being able to kill one of them. Only Lanfear knows that Asmodean is shielded. It's like Semirhage hiring Slayer to kill Mesaana for her - she would consider it a failure from the beginning. Logically, the Forsaken would go after him themselves. Sammael directly says as much to Graendal.

4. Asmodean could kill Slayer in moments, even with his reduced strength. He had enough saidin to battle the Trollocs in Caemlyn, and was about as strong as a Wise One according to Rand. We've seen Moghedien weave in an instant. To think that he'd go pale and cry out in terror at the sight of a non-channeler assassin is laughable. If he ran into one of his peers though, he would know from the beginning that his life was over.

For the record, I say he accidentally ran into Graendal. She got a huge fright and so did he. She balefired him. He would have done the same if he had enough power.