I think you might be interested in reading about the orthogonality thesis, which addresses exactly that. There is very little reason to believe that advanced technology goes along advancement along the scale of (your own) morality axis.

All points of that 2D graph are available.

Edit: also I think you're misreading the Dark Forest concept. They're not saying those aliens are "as bad as [us]". It's rather akin to a prisoner's dilemma. The logic is:

#1. if only one actor is paranoid enough and strong enough, they will proactively get rid of whoever speaks up.

From this axiom comes the logical conclusion that, since we cannot be sure to avoid detection forever, the only viable survival mechanism is to be paranoid ourselves and get rid of others before they become strong enough and can enforce axiom #1.