It is possible, tho tricky, to kill him without triggering combat. However, everyone still becomes hostile.
You can split the party, speak to Halsin and fasttravel away without killing everyone.
Hopefully will be actually stealthable in full release.

When he sits on his throne there is a pit to the right that you can knock him into. Just happened to me on accident.

Used command drop as he got within melee range, picked it up and trivialized the fight

Shame it's literally all incorrect information for the stat table, dude has 75 hp on balanced, not 50, plus his weapon allows him to attack twice.

"Dror Ragzlin will try to damage your character, try to prevent this."

He talks to female characters.