This is one of those questions that can be answered with the Game Banshee walkthrough. I really recommend you give it a look sometime
But, I'm not doing much at the moment, so, in brief:
You *definitely* want Therana alive... Because later on you'll do a quest for her and you can receive a *very* nice reward, depending on how you answer her upon returning.
It can also be helpful to have her alive for the game's Main Quest...
Now... remember those scrolls you were given before embarking on the clothes mission? When Therana attacks... don't return the favour, just use a scroll and get out of there.
Don't worry, when you have to see her again she'll have forgotten all about the incident and she won't bring it up.
Alternatively, you can give the skirt to the nearby slave to try on... at which point Therana will fly into a rage and kill him. Personally, I prefer the former approach, but that is up to you.
Game Banshee Morrowind Walkthrough - Main Quest Side Quests Telvanni