Great Buildup > Great Execution (Satisfied)
Great Buildup > Poor Execution (Unsatisfied)
Poor Buildup > Great Execution (Meh..at least its better now)
Poor Buildup > Poor Execution (Totally Blowed)
Perhaps to make use of a teammate who's mortally wounded?
Or since Tobirama knew about what happens to Uchiha who lose a loved one, maybe it was an attempt to bring that person back from the dead for a time, so they could say their goodbyes or something. Maybe he tried to use it for Madara and his brother Izuna, but it just made things worse.
I think he developed Edo Tensei as weapons. Madara said it was developed to get rid of the entire battlefield in one fell swoop when he dropped a meteor on the Alliance and himself and Muu without a care. It was probably part of the plan when fighting - use powerful attacks to clear as much battlefield as possible. Maybe this is one of the reasons why Senju were considered one of the best clans - Hashirama and Edo Tensei.
As far as morality is concerned, I don't think they ever prioritised it over the security of the village when deciding Edo Tensei to be a kinjitu.
If you look at Itachi's explanation for Izanami, it is labelled as a kinjutu not because it makes you lose an eye (a personal concern,) but because there is a blatant weakness in the technique (which can endanger the success of a mission.)
I think the main problem here with Edo Tensei is the fact that there is a slight possibility the one being raised would be able to free themselves from control like Madara did. And if they decide to rebel, they would be going on rampage in an invincible body with infinite chakra.
Of course, this is the main way I see it for now, there's a chance that we haven't seen the full extent of this technique yet.
why didnt 2nd hokage bring back 1st hokage back .
so i assume the second knew that Hashirama would want to stay dead, so he respected his brothers wishes ?
like Orochimaru said the second should have never written ET down and should have kept it in his mind..I.E. only he knows how to do it
Tobirama most likely didn't bring back Hashirama because it was an unholy jutsu. Not only does it require a living sacrifice, but it also pulls a soul back from the death and messes with time. Tobirama likely chose not to at least use Edo Tensei to bring his brother back out of respect. Plus, it'd totally ruin the idea of trusting in the next generation.
I don't think he invented the jutsu to keep beloved people around after death. It's disturbing for their souls.
Exactly the reason why he labeled it as a Kinjutsu, I'd say.
That and because it required a live sacrifice, and it wasn't humane, apparently. It was also a kinjutsu likely because it'd have become one of the best weapons, especially if the caster was able to control the bodies. Like how the powers in FMA banned human transmutation to prevent armies, Tobirama probably banned Edo Tensei so no one could make an army out of the dead like Kabuto did.
I also considered that, but in that case, Tobirama might have looked for a suitable person to pass the jutsu down. Someone who won't go crazy with the power he'd be granted through Edo Tensei, like Hiruzen, perhaps.
So, that fear might not be the primary reason he forbade its usage, but I totally agree, it was inhuman, so, the most likely option is obvious.
I doubt he'd have passed it down to anyone though, even to Hiruzen. He probably did though, but HIruzen never used it because of his beliefs. We saw what he thought of the jutsu, so while he likely did know it, he never used it. Especially being a man of peace, Hiruzen would not believe in a jutsu like htat, as would Hashirama.
If nothing else, the creation of this jutsu says more about Tobirama than anything, in my opinion. He was probably the darkest hokage and resembled Danzou the most, although not quite selfish.
What is still not explained is Madara's knowledge regarding breaking free out of the contract. This is something Tobirama, who invented the jutsu, Orochimaru, who completed it, and Kabuto, who took it to a wholly new level, don't know about.
Aside from that, the development reason of the Edo Tensei isn't that vague. Tobirama could have had a few more words about it, but he didn't.
Why wouldn't he know about it? Maybe Tobirama does, or maybe Madara merely gambled and rolled a 7.