@sirflimflam : Two things here...
At this stage in the manga (which I later dropped for being complete horseshite), I wasn't convinced she had actually done anything of the sort. This is EXACTLY the sort of manga that engages in deceptive psychological shenanigans, and figured that he'd probably imagined it because he couldn't see clearly what was going on. At least there was sufficient cause for him to be uncertain about what he saw... which would lead one to naturally err on the side of "denial with suspicion".
Regardless, enough time has passed that if anything, fear would usually be motivating him to behave in a less conspicuous manner since he isn't sure what could come of drawing too much attention to himself.
Instead... he is just gawping like a complete retard... ages after the fact. 99% certain he IS brain-damaged here.
As to the other part... if my mother pushed someone off a cliff and I was the only witness... I'd probably just nod appraisingly and say
"Nice." ... maybe a bit envious that she got to kill someone before I did ... But then I'm an extreme misanthrope and want most of humanity dead (my mother isn't, but that isn't really the point).
I mean... I don't even know you, but if you were standing on the edge of a cliff with nobody around, you can bet I'd kick you off and laugh as you fell... just because you were there.