I haven't followed this case closely, but am reading what people have posted on it in here. This was the first I've heard of her mother's partner dying from a drug overdose. That certainly does seem suspicious to me. Was that death investigated or suspected to be foul play in any way? Did the victim have known substance use history? And did Ms Darden receive any insurance money or benefit in some way from her death?
This method of killing a person does seem to me that it could be a way for someone to get away with murder, literally. Obviously, not always, just look at this case in question of Kouri Richins (alleged murderer). But it seems like if you just don't do anything stupid, you might get away with it... might not even ever be a suspect. I would imagine "doing anything stupid" would include the most obvious -- telling someone you did it. Most people find that a very tough secret to keep, imo. Another obviously stupid mistake would be buying/upping life insurance on the victim shortly before their death. What else might lead LE to suspect someone in such a case? Telling inconsistent stories or lying to police, basically just looking guilty? Because absent any actual physical evidence, like a video of someone injecting the victim or forcing pills down their throat or spiking their drink with the drug, it seems like it would be just your word against theirs, and they're dead, so their words will never be heard. People die from drug overdose all the time unfortunately.
What was it in KR's case that led them to suspect her? Because her husband did have a past history of substance abuse, didn't he? But that was in his past, and he didn't at the time he died? I'm curious what it was that made LE start investigating his death as a murder and her as a murder suspect?