edo tensei, created by tobirama, developed and perfected through it's applications by orochimaru and kabuto. the technique that violates the dead and disturbs the living at unconscionable cost.
so, why would a senju hokage create such a foul jutsu that, in the end, was judged immoral and ultimately instill a feeling of regret for its creation?
619 opens it wide open for discussion. orochimaru points out that tobirama's jutsus (contextually, orochimaru means primarily edo tensei but certainly not limited to just that jutsu alone) as well as the skewed applications of ethics and policy onto uchiha caused considerable harm to their clan society as well as spoiling political climates. tobirama openly admitted his treatment was skewed BUT he follows with a defense which is hard to argue against and gives both orochimaru and hashirama reason for pause... MADARA!
"you don't know madara"... "after what madara pulled!"... "madara sympathizers remained"..
let's plug in some more details from the manga prior to this chapter, 619.
1) madara knows the edo tensei contract seal.
2) madara cared deeply for izuna
3) izuna died before madara
4) edo tensei wad introduced in the story as first and foremost, a prelude bargain to return dead loved ones to life that tsunade had lost.
5) tobirama, in transative, is weaker than madara, unable to use mokuton, was alive in the era of opportunity to fight madara.
taking all of that into consideration, one thing is clear... edo tensei was created to handle events having to do with madara!
so, was it used as a means to combat madara in a battle? was it used to bring izuna back to life and appease madara? did madara lose izuna because izuna was the sacrifice in the tobirama's edo tensei? did madara spy, genjutsu, or somehow steal the jutsu from tobirama to bring izuna back himself? did tobirama invent edo tensei to teach madara how to bring izuna back to life? did tobirama create edo tensei for all uchiha as a political bargaining chip during konaha's restructuring and relocation of the uchihas and hoped it would give any uchiha the means to keep loved ones in their world, thus preventing the sharingan from widespread development otherwise caused by severe anguish (imagine the entire uchiha clan during a war! all emos!)??
imo, i think madara learned edo tensei by copying it with sharingan while tobirama used it to bring izuna back from the dead as a means to solve a grave and severe combat situation. after all, tobirama stressed the consequences and transformation that occurs when uchihas lose love and spoke of it in a manner suggesting tobirama had to deal with it at its worst as well as claiming to have seen it many times from the uchiha as a whole.
the whole idea of madara having sympathizers attached to anti-senju/village sentiments means an injustice occurred, izuna died as an injustice madara held konaha responsible for. madara is the reason tobirama created edo tensei, "wiping the slate clean" is open to interpretation.