We haven't really been killing them, just banishing them back into the Fade. I don't remember whether that's established with Gaxkang or Xebenkeck, but point is, we're not some schmuck killing the millenia-old Forbidden Ones. We're giving them a minor setback, at best.
Also, there are theories that suggest we've already met the Formless One. The Band of Three (the ill-fated group in those hidden Kirkwall codices) speculated that the Forbidden Ones = the Forgotten Ones of elven lore. Trespasser confirms that the evanuris considered the Forbidden Ones to be enemies (deserters?), so that's another connection to elves.
If the Forbidden Ones are the Forgotten Ones, we know three names for the Forgotten Ones. Geldauran, Anaris, and Daern'thal. But wait, if they're the same, wheres #4? Well, according to Dalish legends, Fen'Harel was considered kin to both the creators and the Forgotten Ones. And Imshael was oddly buddy-buddy with Felassan, who we now know was an agent of Fen'Harel.
And hey, how strange, in Trespasser we're given the option of whether we want to try to kill Solas. Almost like he's going to be an optional fight in DA4.