That or she isn't sufficiently certain about being able to take us, and doesn't evaluate the benefit as being strong enough, that she's willing to risk it.
Or it's over concerns of how others would react. Oh, and before I forget again.
Okay, I know both of these points have been brought up before, but you're basically arguing that Rionna could be a completely immoral monster capable of faking everything we've seen out of her so far. That's not really something that can be disproven, though I can give some evidence against it.
Suppose that Rionna is concerned about the reactions of others. Whose reactions would Rionna be concerned about? Let's assume that Kyubey or some other actor can show Rionna just deciding to gank Sabrina to literally anyone, casting her as someone just willing to kill anyone. Who might she be concerned about?
1. Her neighbours - with a very broad definition of neighbours including a lot of Western Europe. Edinburgh is prime territory that's hard to hold, and her neighbours are already terrified of her. This might push them into attacking her.
2. Her long term allies: she has to get information about grief manipulators
somehow, and that information dries up if they're afraid of her.
3. Her targets: Who may become harder to hit because of this, somehow.
4. Kyubey: Maybe if she sticks to certain rules he won't screw her over.
5. Her enemies - there's nothing like 'she killed my friend too!' to really get together and wreck her day.
So Rionna goes to talk with Sabrina instead, and immediately starts provoking her in the middle of a bubble she has good reason to think prevents communication outside. This means that either a) she doesn't believe Sabrina / Sabrina isn't correct about the bubble; b) the bubble doesn't matter, because she can employ a magical girl who can read history or detect lies, etc.; or c) it's not actually because she's concerned about reaction to the deed. The first seems unlikely, because the bubble has been pretty consistent on blocking everything. The second is possibly true, but I find it hard to believe any of those possible options would be favourable to Rionna, because any kind of digging would indicate that Rionna had every intention of a fight breaking out there, and it also casts her power in the
worst possible light. She would know this, too.
Okay, so what about power? How would it come about that Rionna wouldn't be sure about winning a fight with Sabrina et al., and how would she react? Well, either Rionna knew that from the beginning, possibly because of the number of grief manipulators she's encountered or because of prior research; or she didn't, and drew that conclusion sometime after arriving in Mitakihara. If it's the former, she came anyway, believing it worth the risk. If it's the latter, she may still be intent on her original goal, or she may be considering backing off.
Any way she's putting on a
very confident impression of being able to take Sabrina in a fight.
-If Rionna still intends to take Sabrina's power, her actions are probably designed to provoke Sabrina, and possibly cause Sabrina to underestimate her (because Rionna is 'arrogant'). What does she do if her provocation fails? Either the power is worth enough to her that she attacks Sabrina anyway, or she backs off to maintain her image (if she's concerned about that). If it's the latter those various hints we have about Rionna's need for this power must all be false, or not as strong as we believed. Or maybe she's playing an even deeper game to make Sabrina lower her guard, which implies she has an almost perfect social mask.
But the thing is - the best opportunity to gank Sabrina was
still at the first meeting. None of the actions Rionna has taken have in any way caused Sabrina to lower her guard or put her in a worse position, unless she's lying about being unable to use her shades. The 'I am a damaged meguca' impression makes a person more wary, not less - and she could also just go along with Sabrina's plans, then gank her later. It would make Sabrina much less suspicious, and work better. We also have no reason to think that Rionna's lying about the shades, because the bubble shuts out
everything we're aware of.
-If Rionna no longer intends to take Sabrina's power, she's backpedalling. In that case there's
no reason for her to provoke Sabrina, unless that's a mistake. And
that means that her composure can be broken by stress - her mask isn't perfect. Alternatively, she's playing the poor damaged meguca role. In that case, again, why wouldn't she accept Sabrina's help, lie, and go her merry way? She may be concerned about image, but that doesn't compare to a good chance of getting killed.
In any case, the reasoning behind Rionna's actions becomes
really convoluted if either or both of those are her motivations with zero moral code behind them.
