fact: Elayne and others describe the palace as labyrinthine but lofty. It has these large corridors, interconnected, with many doors, niches with objects - especially the ground floor where the "public areas" like the throne room, ballrooms, reception rooms, waiting rooms, courtyards and so on are located. This is the area important visitors see etc.

fact: the area Asmodean was in is one used by the Queen and guests. It leads to this nice and quiet inner courtyard she uses, where Aviendha, Mat and Asmodean were. The rooms Rand has picked for himself look over thsi courtyard. The point is: this is not a work area for servants or workers, it's an area of the palace for the masters. Servant places in that area would be located with service to master rooms in mind (ie: discreet but close to the rooms where service is needed).

fact: the corridor from the courtyard Asmodean used was one in which Rahvin and Rand fought.

fact: At no point did Rahvin and Rand ended up in a dead end with a small door. They kept moving from corridor to corridor until Rahvin suddenly went into TAR. The point is: the small door can't be located at the end of a corridor, and the corridor won't be a long doorless stretch either. The small door was along the way in one of the side walls, and the corridor keeps going beyond it. The corridor may lead to the throne room where Rand surprised Rahvin, in fact we know</i it does because of the balefire marks, but we don't know if it does so directly, or you have to turn here and there and take other corridors before you get in that area, and how far it is from the courtyard.

fact: We don't know where Asmodean was heading from the courtyard. Maybe he did, but he didn't think about it. He didn't know the palace well, and was most likely going back in the way he came out to the courtyard. Was he returning to Rand, going back to his rooms (neither would be on the ground floor in Caemlyn), just taking a walk? We don't know. What we know is that the small door wasn't his destination, he didn't even know it existed, and was never looking for something like that! He spotted it all of a sudden as he progressed in the corridor, seing it his mind made the immediate connection to servants (because it's a small door in comparison to the masters' doors around), and to a servants' pantry (which may or not be there.. we don't know Asmodean was right, though the smallness of the door in a palace is a valid indication this door most likely leads to a servant area1/room or another). That door made him think he could use a drink. He stopped, took the last steps to the door, opened it, stepped inside and there, he saw his killer.

So, is it conceivable that someone who saw Asmodean go back in could have guessed where he really went (as opposed to his final destination he never reached and we don't know about, if he even had a precise aim in mind)? No, absolutely not. These corridors lead to many places, and this one is confirmed not to be a deadend. Someone observing Asmodean would make the exact same guesses we can make: that he was heading back to his rooms, or to Rand, or was taking a stroll in the palace. We don't even know if the room he entered really lead to a pantry (Maybe not. Food and drink service in Caemlyn is shown as being done from the kitchens later on, drinks and food being carried in the formal rooms before the guests arrive. It doesn't mean the floor's footmen etc. don't have pantries to await calls or rest, however), and no one goes to the servant pantries normally. People ring a servant who comes to you. Asmodean entered that door because most of the servants had fled and no one was in attendance to serve anyone. To figure out the room Asmodean entered, someone would have to be right behind him in the last seconds before he saw the small door, went to it and opened it. It's very unlikely someone was behind him. That person would have to come from one of the side rooms or the courtyard, to begin with, and with the palace empty and thus silent, Asmodean would have noticed footsteps behind or ahead of him anyway (and before you mention that, Slayer wears boots - not slippers or mocassins - he can't be that stealthy), especially since there'd be a lot of echo in an empty high ceilinged palace (we even know for a fact there is, much later Elayne and co. hear the echo of Slayer's footsteps as he clumsily runs away from the spot he was spying on them).

This is all irrelevant anyway: Jordan said Asmodean was a "road kill" and a murder of opportunity. No one has been following him, waiting to ambush him and kill him. If Jordan wanted us to deduce that, he would have had Asmodean notice a noise and look around at some point in his strolling, before he opened the door. The scene is written quite clearly so we understand right away Asmodean wasn't heading for this location all along but at the last minute chose to open that small door. The person inside the room wasn't waiting for him, the person was surprised by him in this room, but that person was faster to react than he was (unlike Asmodean, the person had a few precious seconds' warning that someone was opening the door and getting in the room).

It doesn't mean you have the wrong killer, but most definitely you have the circumstances of the murder all wrong, because your theory as it stands contradicts the facts laid out by the text.

You want Slayer to be the killer, start by finding believable reasons for him to have been in that room where Asmodean surprised him, and revise your facts to make this into a proper murder of opportunity, not a manhunt and execution you twist into a fake murder of opportunity..