I do not know the Sharee‘ah pretext that Qutuz, May Allaah have mercy upon him, had when killing the envoys. Envoys in Islam should not be killed, and this ruling includes the envoys of the Muslims, disbelievers, and even the apostates.

It was narrated on the authority of ‘Abdullaah ibn Mas‘ood, May Allaah Be Pleased with him, that he said: “Both Ibn An-Nawwaahah and Ibn Uthaal, the envoys of Musaylimah, came to the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, who asked them, ‘Do you testify that I am the Messenger of Allaah?’ They said, ‘We testify that Musaylimah is the Messenger of Allaah.’ On that, the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said, ‘I believe in Allaah and His Messenger. Were I to kill an envoy, I would have killed you.’"
In his comment on this Hadeeth, ‘Abdullaah ibn Mas‘ood, May Allaah Be Pleased with him, said, "It then became the Sunnah not to kill envoys." [Ahmad, Al-Haakim, Abu Daawood and An-Nasaa’i]

It was further narrated on the authority of Nu‘aym ibn Mas‘ood, May Allaah Be Pleased with him, that the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said: "By Allaah, had it not been for the fact that the envoys should not be killed, I certainly would have chopped off your heads." [Abu Daawood and Ahmad]
In his valuable book Nayl Al-Awtaar, Imaam Ash-Shawkaani, May Allaah have mercy upon him, said, "Both Hadeeths (of ‘Abdullaah ibn Mas‘ood and Nu‘aym ibn Mas‘ood, May Allaah Be Pleased with them,) attest to the prohibition of killing the envoys of the disbelievers, even if they said the word of disbelief in the presence of the ruler or the other Muslims."

The Sharee‘ah ruling in Islam then is that no envoy should be killed. I do not know on which pretext, under Sharee‘ah, Qutuz, May Allaah have mercy upon him, depended when he killed those envoys. I do not know also why the scholars at his time kept silent on this issue. They might have talked about it, and no report was handed down to us concerning that. It may be also that in the personal reasoning of Qutuz, May Allaah have mercy upon him, the Tatars had violated all conventions, deeds and laws, and killed non-fighting women, children and old people in countless numbers, and it is sufficient to say that in Baghdad alone, they killed over one million Muslims, let alone the preceding and succeeding towns. It could be that he endeavored to reach this conclusion after the envoys had behaved impolitely and arrogantly towards him, and spoken harshly with him.

That was his personal reasoning. I still see that we should not be dragged to the filth of politics and the bad morals of the disbelievers. For example, if the disbelievers killed Muslim children, it is impermissible for the Muslims to retaliate by killing the children of the disbelievers under the pretext of equivalent punishment; and if the disbelievers killed and violated the honor of Muslim women, this should not justify killing and violating the honor of non-fighting disbelieving women, given that it is unacceptable to violate the honor of the women in general, be they fighters or non-fighters; and if the disbelievers betrayed the treaties, it is impermissible for the Muslims to betray theirs, similarly, it is impermissible for the Muslims to kill the envoys or the disbelievers who are granted safety by the Muslims.

All of this is intended so that the religion of Islam would remain clean and pure, uncontaminated by the filth of positive laws, and so that the Islamic supreme ideal and the Islamic high moral character would remain an enduring means to invite the people of the earth to this wonderful religion, i.e. Islam.
In view of the previous reasons, I consider the behavior of Qutuz, and the emirs in this respect, to be a flaw, and incorrect personal reasoning. To be sure, everyone, no matter how much he is held in high esteem, falls into error, save the infallible Prophets.
If one’s defects could be easily counted, then that should be a sufficient source of pride for him.

https://en.islamstory.com/index.php/en/artical/20790/pause-reflection