Yeah. It's easy for us as the readers to go "why is Tsukina such a weak, wishy washy character!?", but honestly that's way more realistic and believable than her becoming a super confident badass overnight.
Wow. This is looking super bad.
Problem is, instant death magic is boring. Unless you go the comedic route (eg, the other Instant Death series) there isn't really anything interesting you can do with this concept.