In such a case, doing ethics would just be descriptive, it would tell us nothing new, and the outcome would be whatever evolution arbitrarily equipped us with

I used to worry about that a lot, and then AndrewCritch explained at minicamp that the statement "I should do X" can mean "I want to want to do X." In other words, I currently prefer to eat industrially raised chicken sometimes. It is a cold hard fact that I will frequently go to a restaurant that primarily serves torture-products, give them some money so that they can torture some more chickens, and then put the dead tortured chicken in my mouth. I wish I didn't prefer to do that. I want to eat Subway footlongs, but I shouldn't eat Subway footlongs. I aspire not to want to eat them in the future.

Also check out the Sequences article "Thou Art Godshatter." Basically, we want any number of things that have only the most tenuous ties to evolutionary drives. Evolution may have equipped me with an interest in breasts, but it surely is indifferent to whether the lace on a girlfriend's bra is dyed aquamarine and woven into a series of cardioids or dyed magenta and woven into a series of sinusoidal spirals -- whereas I have a distinct preference. Eliezer explains it better than I do.

I'm not sure "intriniscally awful" means anything interesting. I mean, if you define suffering as an experience E had by person P such that P finds E awful, then, sure, suffering is intrinsically awful. But if you don't define suffering that way, then there are at least some beings that won't find a given E awful.