Surah Al-Ma'idah (5): 70 - Laqad akhadhna mithāqa...
It is understood that the purpose here is to explain the arrogance of the Children of Israel and the intensity of their rebellion against fulfilling the covenant of God, which relates to the opening of the Surah: {Fulfill the contracts} (Al-Ma'idah: 1).
God states: {We have certainly taken the covenant of the Children of Israel} (5:70). This means We established the proofs, created the intellect guiding them to the method of inference, and sent messengers to them to define the religious laws and rulings.
The phrase {Whenever a messenger came to them with what their souls did not desire} is a conditional clause describing the messengers. The returning pronoun is omitted. The meaning is: Whenever a messenger came to them (from among them) with what contradicted their desires and opposed their lusts, such as the hardships of religious obligations.
Here are some questions:
The First Question: What is the response to the condition?
The statement {a group they denied, and a group they killed} cannot serve as the response to this condition, because a single messenger cannot be two groups.
The Answer: The response to the condition is omitted. Its omission is permissible because the mentioned speech serves as evidence for it. The intended meaning is: Whenever a messenger came to them, they opposed him. Then it is asked: How did they oppose him? It is answered: {a group they denied, and a group they killed}.
Regarding the objection that a single messenger cannot be two groups: We say that the phrase {Whenever a messenger came to them} indicates the plurality of messengers, so it is natural that they were divided into groups.
The Second Question: Why is one action mentioned in the past tense, and the other in the present tense?
The Answer: God Almighty explained how they used to deny Jesus and Moses in every situation, and how they rebelled against their commands and obligations. It is mentioned that the Prophet (Moses) died in the wilderness according to some opinions due to the ill omen of their rebellion against accepting his command to fight the tyrants.
As for killing, it is what happened concerning Zechariah and John (peace be upon them). They had also intended to kill Jesus, even though God prevented them from their goal, yet they claimed they had killed him.
Mentioning the denial with the past tense here refers to their dealings with Moses (peace be upon him), as many eras had passed since that time. Mentioning the killing with the present tense refers to their dealings with Zechariah, John, and Jesus (peace be upon them), as that time was closer and thus felt like the present.
The Third Question: What is the benefit of fronting the object in the Almighty's saying: **{a group they denied, and a group they killed}**?
The Answer: You already know that fronting (placing the object first) is done for intense emphasis. Although denying and killing prophets (peace be upon them) are both reprehensible acts, denying and killing the prophets is more heinous. Therefore, the fronting serves this purpose of emphasis.
{And they thought there would be no trial [or punishment], so they became blind and deaf. Then God turned to them in forgiveness; then many of them became blind and deaf again. And God is Seeing of what they do.} (5:71)