well, the lack of surprise may be attributed to the fact that she was in fug she got to know who the new slayer candidate is. she is sharp and deduced it was baam. her expression when she met baam was 5-6 yrs after the act she did. Also killing Bam isn't the only bad thing she did she killed akraptor and prince and koons other teammates stabbed dan, partied with white knowing that he will most likely kill bam (since we are talking about expression her expression when she realized bam defeated hoaquin was enough to show what she wanted).
Suddenly she just jumped in to rant about how she is the savior and Destiny is BS.(when de shah didnt even name her the savior he said she had ominous aura around her, Bam is the one who requested on her behalf), She will not rest till folks dances around her and hail her as the savior and she gets to see what she wants to see and to do that she will manipulate destroy lives and use every vile way like killing a child who promised to be her legs and defend her.
Let's just revisit S1, Baam had 0 incentive to prat around saying I am irregular thus marking him decisively for FUG and the Zahard corps. None of the two options were good for him. He, of course, didn't know that his life is now marked for danger but he was keen and perceptive enough to realize being irregular will bring him unforeseen complications, yet he loudly declares he is irregular just so he and rachel can take the test and go up so that rachel can see the stars(whats in it for bam? nothing, yet he is selfless enough to do that). Then she just stands up because she calls BS to destiny and without any remorse or doubt push that boy into a chasm come out pretend she is down without shedding one single genuine tears at what she did.
Yep If that means to BS destiny I d rather embrace destiny