Why are the Zen-Ohs going to destroy the universes that lose in the tournament of Power ? It seems rather brutal and pointless, and make the Zen-Ohs look evil just for the sake of being evil.
By now the anime has provided a clear answer (though if you're only following the broadcast of the dub you're not quite to it yet):
The whole tournament and super dragonball wish was a secret test of character. Wishing back all of the universes was essentially the only correct and allowed wish. If a selfish wish had been made, they were going to take it as proof that those Universes are, in fact, not worth keeping, and all of them would be destroyed. But wishing them back would be a testament to the caliber of the denizens of these universes, such that they would be returned and spared.
Oddly, this seems to be a little different in the manga:
None of the "secret test of character" stuff gets brought up at all. Seventeen says he wants to wish all the universes back if the super dragonballs can do so, and the Grand Minister confirms that they can. So they're wished back and that's pretty much the end of it. All that's said is that the wish elevated Universe 7's rank by one position. Which does at least say that such a wish was (highly?) valued, but not nearly at the level it seems to have been in the anime.
It's being stated that the Zeno samas want to destroy the universes which have low average mortal level and the 8 participating universes have the lower mortal levels . Why they want to do destroy such universes with low mortal level, it remained unexplained.
May be they'll give a twist to this idea somehow in the future and the universes will be resurrected, falsely destroyed or something, but for now it's being stated in a japanese magazine than (spoilers)
universe 9 will be the first one to be destroyed