Since the days in Gurukula, Karna was jealous of Arjuna. This led to him hating the Pandavas and making friendship with Duryodhana
“Then the valorous Drona taught Pandu’s sons the use of many weapons, human and divine. O bull among the Bharata lineage! Other princes also came to Drona, supreme among Brahmanas, to learn the use of arms—the Vrishnis, the Andhakas, kings from many countries and Radheya, the son of the suta. They made Drona their preceptor. The suta’s son was envious of Partha and always competed with him. With Duryodhana’s support, he showed his contempt for the Pandavas.”
- 122, Sambhava Parva, Mahabharata.
Karna was part of the planning to kill Pandavas in Lakshagraha (Vrisha is Karna)
When the Pandavas were not killed through all these means, open and hidden, since they were protected by fate and destiny, he consulted his advisers: Vrisha, Duhshasana and the others. With Dhritarashtra's consent, he had a house of lac built.
- 55, Adi-vamshavatarana parva, Mahabharata.
Karna laughed at Draupadi as Dushasana dragged her into the assembly
Duhshasana dragged her with even greater force, so that she almost lost her senses. He repeatedly called her “slave” and laughed uproariously. Karna was delighted at these words and approved of them by laughing out loudly. In similar fashion, Soubala, the king of Gandhara, applauded Duhshasana’s deed.
- 60, Dyuta parva, Mahabharata.
When Vikarna tried to prove that Draupadi was infact not a slave, Karna countered him saying that Draupadi is a courtesan for having 5 husbands
O descendant of the Kuru lineage! It has been ordained by the gods that a woman should only have one husband. However, she submits to many and it is therefore certain that she is a courtesan. It is my view that there is nothing surprising in her being brought into the sabha in a single garment, or even if she is naked.
- 61, Dyuta parva, Mahabharata.
Following this, Karna is the one who ordered Dushasana to strip Draupadi naked
O Duhshasana ! This Vikarna is only a child, though he speaks words of wisdom. Strip away the garments from the Pandavas and Draupadi."
- 61, Dyuta parva, Mahabharata.
Karna told Draupadi to enter Kuru household as a slave and serve the Kauravas
Karna said, “There are three who can own no property—a slave, a student and a woman. O fortunate one! You are the wife of a slave and have nothing of your own. You have no lord and are like the property of slaves. Enter and serve us. That is the task for you in this household. O Princess! All the sons of Dhritarashtra are now your masters and not the sons of Pritha. O beautiful one! Choose another one for your husband, one who will not make you a slave through gambling. Remember the eternal rule among slaves. Sexual acts with one’s masters are never censured. Nakula, Bhimasena, Yudhishthira, Sahadeva and Arjuna have been won over. O Yajnaseni! Enter as a slave. The ones who have been won over can no longer be your husbands. Valour and virility are of no use to Partha now. In the middle of the sabha, he has gambled away the daughter of Drupada, the king of Panchala.”’
- 63, Dyuta parva, Mahabharata.
Karna was part of Duryodhana's plan to capture Krishna
They decided on a course of action Duryodhana, Karna, Shakuni Soubala and Duhshasana as the fourth. “Janardana is swift in his action. Before he captures us, with King Dhritarashtra and Shantanu’s son, we will forcibly capture Hrishikesha, that tiger among men, like Indra seized Virochana’s son.
- 128, Bhagavad Yana parva, Mahabharata.
On 14th day, Karna had no issue attacking Satyaki who did not have a chariot then (Yet this same guy hypocritically called on Arjuna to not attack him when his own chariot sunk to the ground)
On seeing that Satyaki was without a chariot and Karna was attacking him with raised weapons, Madhava blew a rsabha note on his conch shell with great force.
- 122, Ghatotkacha vadha parva, Mahabharata.
Krishna termed Karna as the root of all of Duryodhana's wicked deeds
O Arjuna! Whether it was the attempt to burn down your mother with her sons in the night, or whatever Suyodhana attempted towards you in the course of the gambling match, the evil-souled Karna was the root of all that. Suyodhana always thought that he would be saved by Karna and angrily tried to seize me too. O one who grants honours! It is the firm belief of that Indra among men, Dhritarashtra’s son, that Karna will certainly defeat all the Parthas in battle. O Kounteya! Though Dhritarashtra’s son knew about your strength, he found pleasure in a conflict with you because he depended on Karna. Karna has always said, “I will defeat the assembled Parthas, Vasudeva and the kings in the great battle.” The evil-minded one has encouraged Dhritarashtra’s evil-souled son and roared in the assembly hall. O descendant of Bharata lineage ! Kill Karna today.
- 51, Karna Vadha parva, Mahabharata.
Sanjaya severely criticized Karna as a wicked man who gave wrong advises to king Dhritarashtra and Duryodhana
You were avaricious and desired the fruits and did not act in accordance with what was good for you. Your advisers were Duhshasana, the evil-souled Radheya, the evilsouled Shakuni and the evil-minded Chitrasena.They were thorns and they made the entire world full of thorns for themselves.
- 1, Vishoka parva, Mahabharata.
As you can see, Karna was an active participant in Duryodhana's misdeeds and it's because of his constant encouragement that Duryodhana even dared to wage war against the Pandavas.