While I am not a seasoned android user, I've heard from many many people (and personally agree) that killers are useless and sometimes even harmful in most situations. Your phone comes with Task Manager, which is perfectly capable of killing necessary apps if you want to free up some cpu, and a RAM clearer as well just in case. It is less extensive than the killers you can download, but while the stock apps on your phone can only be killed with a downloaded killer, they're just going to start right back up. This is not necessarily a bad thing; it is the way android was programmed to operate. You're better off not getting one.
The only way to completely remove TW is to root your phone. You can do your own research to decide whether that option is for you or not (you have to be fairly tech savvy in most cases). I personally have launcher pro and am in love with it. It just masks your TW UI but it hasn't slowed my performance at all. In fact, its even smoother than before.