Ethical concerns regarding the review of a paper on autonomous armed drones
I suggest you withdraw and send a formal letter to the editor expressing your concerns. Cite a larger moral universe than the narrow one covered by the journal's policies. Suggest that the research itself is unethical and recommend, without detailed review, that the paper be rejected. 

I've been in a similar but less fraught case. I was shepherd of a paper that crossed a less dangerous boundary. I had repeated contact with the authors and tried, over several communications, to convince them that the actions they were proposing were wrong. I failed to convince them. The paper was rejected. I was actually given an award for my efforts to convince them, even though I failed. 

My experience suggests that the people who do this sort of thing are not going to be convinced by a dialog of any kind. They are too invested in it to bother to listen. 

Don't wait for "the powers that be" to decide this stuff is immoral and leads to a bad end. If people like reviewers at the bottom of the pyramid don't complain about it, this sort of research will continue and will get implemented. There are too many historical exemplars of this to ignore the consequences.