Should one cite unethical research that may contradict one's current research?
Are you conflating evil people with scientific results that they achieve (by valid means)? They should not be conflated. Condemn the people and the evil acts that they do, but not any valid results obtained by honorable means. Evil monsters don't need to always act in an evil manner. 

There is, for example, a lot of both computer engineering and mathematics that was done by total racists. This was in the US and in fairly recent years. This doesn't make it invalid to use computers, nor does it invalidate the mathematics itself. 

It isn't even necessary to state that X was an evil monster otherwise when citing X. However, if you are writing about racism or other evils, then it is perfectly appropriate to point out the flaws. 

Of course, the spline research would be discredited if to do it evil acts needed to be carried out, but you don't seem to be saying that here. 

What is true is true, even if uttered by monsters. What is false is false, even if uttered by angels. 



Since the question was changed, I'll add this. If "research" has been discredited for any reason then there is no need to cite it other than as something not to be done. There is no need to cite failures to prove a theorem when you provide a proof, and there is no need to cite research that was done unethically. The results themselves are in question. 

There are a lot of ways to fail at research. Lack of ethical behavior is only one. Don't cite the failures, except as failures. 

In the case at hand, the research resulting from unethical behavior doesn't "contradict" the current work. It wasn't valid in the first place.