I take it as axiomatic that Facebook is an unethical actor. I believe I have good reasons for this. From there, I say that anyone who works to make Facebook more effective at its unethical goals is similarly unethical.
No, this doesn't include people that contribute to OSS that originated at Facebook. Improving OSS is a contribution to the community, of which Facebook is unfortunately permitted to benefit from. Facebook starting an OSS project does not make the OSS unethical, because the OSS is a tool.
The question is different for employees because of this community/Facebook divide. Facebook employees are not unethical because of the tools they use for Facebook, they are unethical for furthering Facebook's unethical aims.
On the topic of open source, would it be possible to construct a license that removes the ability of bad actors to use it? Of course, a bad actor could use it against the terms of the license, but the intent of the software project is made clear.
I agree that the sense of which you speak is a nonsensical perspective; I'm not interested in laying blame on a tool for how the tool is utilized.
The Facebook employee isn't unethical because of their nebulous entanglement in nth-order externalities, whereby bad-person X did bad-thing Y therefore employee Z is guilty by association.
The Facebook employee is unethical because of their direct, active, engagement in developing the actual tools actually utilized by none other than their employer for the explicit purpose of making our world a more user-hostile place.