O.K., normally, wearing the cowl and killing somebody doesn't count against you, but every time I try to kill S'drassa in the Leyawiin mages guild, I get exiled from the guild. What's going on? Isn't wearing it supposed to blame everything on the Gray Fox? Why is my PC getting blamed? And I really want to kill S'drassa so that I can get his unique robe and hood.

Yeah sorry but it doesn't work that way. You can't commit any crimes towards your guild and get away with it even while wearing the cowl. As for his robe and hood, I think you can get those elsewhere but I'm not sure where. Try looking in the UESP

You can kill Francois Motierre (Chorrol) for a copy of the robe S'drassa wears. There's other people who wear it but two are in the guild (S'drassa and J'Skar) and one who lives in an awkward place (Lithnilian, lives in the Imperial Bridge Inn on the northern bank of Silverfish River.

Unfortunately, S'drassa's hood is unique to him unless you have Shivering Isles, then it and the robe can be found in a prison cell in Cann.

If on the computer the console id for the robe is 00071019 and the hood id is 0007101A. If you have installed Shivering Isles, one is located in a prison cell in Cann, Arena Substratum. The robe can also be found here Shivering Isles adds two more: Traelius wears one, and one is located in a prison cell in Cann, Arena Substratum.

Hope this helps........

Thanks to everyone who answered, I knew that a set could be found in SI, but I didn't feel like starting that story line yet, so I've been trying different ways to kill him without getting kicked out. It's annoying, he won't eat a poison apple, the guards who chase me as the Gray Fox ignore him/can't kill him, and he won't equip the "death hood" I made.

Maybe one option also is if you cast command human on him and than stab or shoot him with poison????

Haha! Got him! Thanks for the poison idea! Just used my weakest arrow with a poison that does 7 damage over 20 seconds while he was sleeping. The only problem I had was keeping him from running outside, because apparently once he runs outside, the poison stops working. So I just shut him in a room and block the exit! Sure I can't wear the Crusader's armor anymore but hey, you win some you lose some. Just gotta do a Wayshrine circuit again.

Glad it worked out but that idea came from a post I read from Acadian.....

What was his post for/what did it say?

http://www.gamesas.com/index.php?/topic/1193494-how-can-you-kill-someone-without-being-detected/page__p__17740200__hl__%22kill%22__fromsearch__1#entry17740200 Here is the link

Thank you good sir and or madam.

Not a problem........

A way you may have been able to do it is sneak a weightless, enchanted black robes and mages hood into his inventory (reverse pickpocket). Enchanting them with 5% shield or something. After he wears those items, then all you have to do is pickpocket the original robes and hood you wanted without resorting to killing him.