Trying to look at it from a different side.
Liz had life before she married Sergio, she lived in FL, and to me it seems that she was an independent person. She might have had a personal enemy, a person hating her, and especially if that person knew about the life insurance, there was no better way to make this case never solvable.
If a woman lives alone, the first line of suspects is her intimate partner; if there is no such person, then, her relatives and family and then her circles of friends.
But if there is a husband and a life insurance, he is automatically the first suspect. It takes years to clear him and in a way, there is always a cloud of suspicion hanging around him in the court of public opinion.
That arithmetically close timing before his departure and the murderer’s arrival doesn’t provide him with an alibi. The opposite: it casts a suspicion on SB…but what if this is exactly what the murderers wanted?
So what happens: there can’t be two task forces about the case. Logically, everyone is suspecting the husband, and even if his alibi stays, there is always this “what if?” question. So he and his alibi distracts police’s force and attention.
It takes time. And the longer the time, the less chance of the case being solved.
JMO - an absolutely theoretical situation. But just hypothetically, imagine that there might be other people with own financial gain. (An example: In any family there potentially can be an eccentric millionaire grandfather and Liz being one of five people standing to inherit a lot from him. That fact, theoretically, would be totally obscured by her husband and his gain.) No one is looking further than her husband. Ideal for the killers.
At least S. mentioned a woman disliking Liz. People who say “everyone loved her” are either unaware, or don’t want to give information. No one lives in vacuum, and we all have people disliking us. The trick is to make this comprehensive list.
Someone mentioned MO’s killing. I think KA thoughts were: they will either blame it on robbers, or if not, CS was the last person to see Mo. KA’s execution was bad, so she got caught.
The person who killed LB might think the same, they’ll blame it either on random robbers, or the husband.