I agree with Dulan's reply. We should heed what “Leave that which makes you doubt” means - that is to refrain from that which you are uncertain. The following example of a "Analogy of Serial Killer" will show you how we can knowingly or unknowingly be involved in 'killing' kamma.
Suppose we have a serial killer in a certain city who has raped and killed many women so that no woman dares to venture outdoor at night. The whole city is in uproar and the citizens demand that the authorities do their duty and catch the killer. So the police, after several months of pains taking effort, finally nabs the culprit. After this is a long trial and then the judge passes the death sentence on him. On the appointed day the killer is led to the execution platform where the executioner pulls the lever to end the killer’s life.
All this now leads to the question: “Who is involved in the evil kamma of killing a human being (i.e. the serial killer)?” According to the law of kamma-vipaka, the executioner bears the heaviest offence because he intentionally carried out the killing. Next would be the judge for pronouncing the death sentence. These two persons are directly involved in the killing kamma of the execution of the serial killer. The police are only indirectly involved and not responsible for the execution. How about the citizens? Ultimately the serial killer was executed to protect the citizens, i.e. he was executed for the sake of the citizens, or the citizens were the main beneficiaries of the execution. So are the citizens responsible for the killing kamma involved? No, because they did not ask for the execution of the serial killer. But they could be if they demanded his execution. The scenario is similar to the slaughter of animals for food.
The persons who slaughter the animals bear the heaviest killing kamma. The persons who breed animals for slaughter are also involved in the killing kamma. They are like the judge who condemned the man to be executed. But the people who buy the meat of animals already slaughtered are not involved in the kamma of killing even though, like the citizens of the city above, they are the main beneficiaries. But if someone orders a live animal to be slaughtered for its meat, then killing kamma is involved for him.