Why did Darlene put fentanyl in the drugs for the cartel?

So after a quick rewatch of the season, what Darlene thinks she is gaining, is setting up the Mexican Drug Cartel for failure and prove that they can't take advantage of them, as some of their clients become ill and even die after taking the drug, which also then blows back onto Marty and Wendy. Most of their clients probably don't know that the Snells are one of their suppliers, however the FBI quickly figures that out, because of the location of where people dying are (Chicago and the Ozarks) make it easier for them to pinpoint the source.

2.08 Transcript

DARLENE: Is something bothering you?

JACOB: As a matter of fact there is.

DARLENE: Should I make you some cocoa?

JACOB: It was you wasn't it? You spiked the product.

DARLENE: What if I did?

JACOB: You know how many people you've hurt?

DARLENE: Ugh! Some junkies who were killing themselves with dope succeeded. So what? This is about respect. The cartel thought they could roll right over us. Well, they don't anymore.

JACOB: I think you misconstrue the situation.

DARLENE: I told you not to get involved with them.

JACOB: I don't answer to you, Darlene.

DARLENE: I think you misconstrue the situation.

JACOB: They'll come after us. There'll be a war.

DARLENE: We've had to fight before. I am not afraid. Are you?

JACOB: Yes, I am. I am afraid. Of you and your implacable rage. Your rashness.

DARLENE: You killed Ash for no reason. For money, for a casino.

JACOB: I killed him to save you.

DARLENE: I guess we just see things differently.

One has to remember from the beginning that Darlene never wanted to make this deal, but becomes furious when Dell comes to their house, as she finds his manner completely disrespectful. Even though she acknowledges that killing Dell may have been a little rash, she continues to use the argument of disresepct in conjunction to "The Snell way". The more immediate reason Darlene put the fentanyl in the heroin was reprisal for Jacob trying to make amends to the Cartell once they learned about what really happened Dell, when he killed Ash. Darlene viewed Ash as family and though his death was unnecessary, despite they would of all died otherwise. Darlene is not a logical person, which become more evident when she begs god to save Jacob after he was shot, only to turn around in the next episode and

kills Jacob herself!