That they may bear their own burdens [i.e., sins] in full on the Day of Resurrection and some of the burdens of those whom they misguide without [i.e., by lack of] knowledge. Unquestionably, evil is that which they bear.
The Destruction of the Disbelievers and Intensification of their Punishment for rejecting the Revelation
Allah informs us that when it is said to those liars,
مَّاذَآ أَنزَلَ رَبُّكُمْ قَالُواْ
("What is it that your Lord has revealed" They say,) not wanting to answer,
أَسَـطِيرُ الاٌّوَّلِينَ
("Tales of the men of old!") meaning nothing is revealed to him, what he is reciting to us is just tales of the men of old, taken from the previous Books. As Allah says,
(And they say: "Tales of the ancients, which he has written down, and they are dictated to him morning and afternoon.") (25:5) i.e., they tell lies against the Messenger and say things contradicting one another, but all of it is false, as Allah says,
It will be recalled that, after describing the blessings of Allah and proving that He was unique in the creation of the universe, the text made a pointed reference to how the polytheists chose to stick to their own ways of error. In the present verses, the text describes how they, not content with their own straying, tried that others should also go astray like them. The punishment they would get for doing that has also been stated here. Earlier to that (24), there is a question about the Qur’ an, and the addressees of the question, at this place, are the polytheists. It is their haughty and ignorant answer which has been mentioned here along with a warning of consequences for this attitude. Then, after five verses (30), the same question has been addressed to believers who fear Allah which is followed by their answer, and where a promise of reward for it has been mentioned.
It has been recorded in traditions that when the Prophet Muhammad declared his prophethood in Makkah, the news gradually spread and reached the other Arab tribes. Whenever they met the chiefs of Makkah, they used to ask their opinion about the man who had claimed to be a prophet. In reply to this, the chiefs of Makkah used to cast doubt upon his personality and his discourse. (Tafsir al-Mazhari). One of the ways of doing this was to distort or twist whatever was said. For instance, there is some mention of prophets in the Quran. This they could have referred to as ‘the history of previous prophets’, but they called it ‘the stories and tales of the ancient people’. Turning people away from the call of Truth or making them doubtful made them guilty of the worst crime in the eyes of God, because not only had they themselves gone astray, but they had become the means of misguiding others.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.