That is a tough stance. Perhaps these may be applicable.
I read somewhere that it can take 5 “whys” to get to the root of a situation. It was aimed at taking down the strongholds in our own lives. Could it also be a key in these conversations? An author who teaches moral reasoning teaches that, at least in his research and experience, the word “Why” shuts down open conversation. He suggests asking in other ways, like
“What brought you to that conclusion?” or
“How did you come to think that?” Or, perhaps in this case,
“Can you give me an example of that?”
In a single conversation it may not be possible to ask 5 consecutive questions without them feeling defensive, but, maybe over time. It is likely that many have no reason other than “everybody knows that” or they read it in a book, which leads to another question: “What gives you that impression”, or "What leads you to think that author is trustworthy or knowledgeable?
In some cases, it may be helpful to challenge them to read His Word for themselves.
I wholeheartedly agree! Next, I would consider asking questions about evil, targeting those things which were done by the cultures which were destroyed.
How do you feel about people who burn babies alive?
What do you think should be done to people who do this routinely?
What if these people not only did this, but believed it was a morally good act and aggressively persuaded others to also do this?
What if they refused to be convinced that this was not a morally good act and continued to aggressively encourage others to do it?
These answers hopefully lead to some common ground.
Most of the people you’ve mentioned at least acknowledge there is a deity or a possibility of one. Along that line, I might attempt to give them insight into His point of view. We are His created ones. He calls us (those who love Him) His body. With that idea, you might ask questions along these lines:
Do you love your body?
Would you deliberately hurt yourself?
How would you describe cancer? What is cancer to you?
What are traditional treatments for cancer?
If you had cancer, what would you do?
Ideally, they would give an answer similar to the one given by Cancer.org
see headings What cancers have in common and Treatment.
Summary - Cancer cells crowd out normal cells so the body cannot work properly. Left untreated, it can end in death. Treatment requires killing or removing the cells from the body.
Conclusion: Regardless of how much one loves their body, it is an act of love and mercy to remove or kill their own cells, if they are cancer, in order to save their body/life.
Or, if they would not do traditional cancer treatment,
Would you cut off your leg for no reason?
Would you cut off your leg, if it had an aggressively spreading gangrene or other infection that threatened your life?
Might a loving deity remove an evil culture that the rest of mankind might live?
Just some thoughts to get you started.