Absolute Anime

I read while looking around that Tsunami killed off an entire precursor race, presumably over the mistreatment of Washuu, (who being supposedly of immeasurable intelligence it was her own fault for not evading capture). Did she really kill off the whole race, including the little baby precursors who had nothing to do with this incident?

And why did she decide on merging with Sasami? Didn't Z harp on Tenchi once about the Choushin being able to ressurect Tenchi however many times they wanted? Why not simply revive Sasami? I want to see if Sasami had an NDE (just kidding.)

How exactly did this happen?

EDIT: I should probably capitalize "Choushin" - proper grammer.