Will Elesh Norn kill creatures if I don't pay the extra cost for Flash?
No, Elesh Norn's static abilities have no effect on the battlefield in that case.

Bringing Elesh Norn into play with Flash and deciding whether or not to sacrifice it is all part of Flash's resolution. Killing creatures with her -2/-2 would require state-based effects to be checked, but that only happens after Flash has finished resolving, at which point Eles Norn is already gone. 

If you sacrifice a creature brought into play with the spell Flash, basically all you get are "enters the battlefield" and "leaves the battlefield" effects.