It never can do good.
That's a willfully incorrect statement.
Hordes of Darkspawn intent on killing and brutally raping anything in their path is undeniably bad. Using a tool that would reduce the lives lost to such a horde is undeniably good. The methods that this tool employs may also be bad, but it does result in good. Undeniable good. Dead Darkspawn = better lives for everyone.
The Warden relinquishes control of the Anvil and he hands it over to Branka who's outright crazy.
This is true. But she is crazy with a desire to save her people. She sacrificed her entire house - family, friends, servants - to find a means to save the rest of Orzammar in the long term. She acted coldly, but rationally. She isn't using the Anvil to stage a coup against the squabbling kings to take power for herself, she only does what she feels is absolutely necessary to fight the Darkspawn. That's what, arguably, the Warden should be doing the entire game.
Isn't it implied in the ending slides that they're still trying to resurrect it.
Yes, it was also implied through the Golems of Amgarrak (sp) DLC that this would happen regardless of if the Anvil was destroyed or not. In an attempt to avoid using soul slavery, they wound up releasing a much more terrible threat to the world through the Harvesters. Good intentions and paths paved to somewhere and all that.
He was the expert on the Anvil and what it will eventually turn into again, Dwarven nature doesn't change.
The dwarves will change if they are extinct. Which is how the choice is framed - destroy the Anvil and risk certain doom for the dwarves or preserve it and risk the chance for it to be abused. Power corrupts, but death is final.