Well if your study involves depressed people unknowingly being manipulated with negative newsfeed items that they didn't know they consented to being subjected

Well if your study involves depressed people unknowingly being manipulated with negative newsfeed items that they didn't know they consented to being subjected to in a secret study, then that's unethical. The scientists aren't stupid, they knew beforehand that in a sufficiently sized population, there will be a segment of depressed people that they are influencing to commit suicide by being subjected to increased negative newsfeed material. That is unethical to me any way you cut it.