Indeed, as pointed out above, there are any number of factions within the Remans, including Obisek's. I must commend the Devs and writers in not lumping everything and everyone together. They have created shades of gray.
Including my choice: I did not arrest Obisek. As stated in the mission text, "I may not like" what you are doing, but I won't stop you. Utilizing weapons such as thalaron weapons leaves a very strong bitter taste in my mouth, and I seriously had problems with my choice, but ultimately I let him go. Why? Well, slavery evil very narrowly trumps biological warfare evil, ethically, in my mind. But only just, and only when (I hope) Obisek is using those weapons against military targets.
That "I hope" clause leaves me with a very bad feeling of dread. (And with that, I again commend the writers and devs for having such an interesting character and story going in an online game.)