A person never wants to become villain, it's the situation makes them.
'Suryodhon' goes to Bheem' with Kheer , he only knows that it has something that will make Bheem' weak during the fight. But he doesn't know that it has a colourless, odourless mortal poison that can kill him and also has paralysing poison.
He woke up Bheem' and gave him the Kheer, Bheem' happily ate it , after eating he began to go aggressive towards 'Suryodhon' which resulted fighting between them. While fighting Bheem' begins to weaken, and 'Suryodhon' pushed him at the time Bheem' also caught his hand both fall in the Ganga river, due to medicine Bheem' paralyzed and loose consciousness. He began to drown in the river.
'Suryodhon' comes up from the river, trembling in fear he looks at his hand and shouted for help.
Karna' get his clothes wrapped in his head to hide his identity and dives into the river to save Bheem', he knows that in this confrontation Bheem' is guilty, but it doesn't give right to kill him.
Karna' thought that the second Pandavs', tormented his cousins. Bheem' was stronger than all his brothers put together. So, he would wait till the Kauravs' had climbed a tree, then shake it till they fell out and then, laugh at them. Or, he would dive into the river when they were swimming and pull them underwater till they were half drowned.
Though the other Pandavs' were not bullies like Bheem', they overshot the Kauravs' in their lessons on statecraft, warfare, handling of weapons and so on. As a result , 'Suryodhan's jealousy knew no bounds. He wanted to defeat such a way that Bheem' will have trama as then, only would he get rid of his tormentor,
'Suryodhon' invited his cousins to a picnic on the banks of the Ganga. While the elders ate, drank and chatted, the younger crowd played games, swam, climbed trees and enjoyed themselves. Bheem' lost no opportunity to harass his cousins and this strengthened 'Suryodhon' s resolve to get rid of the tormentor.
Unconscious Bheem' sank into the waters, behind him Karna' is chasing him, the 'Nagas' guards bitten Bheem' as he reached the kingdom of the 'Nagas' or serpents who lived in the river. Outside of the Kingdom Karna' requested to meet their King this 'Nagas' guards let him go and inform their King about a intruder and a Visitor. Their king, 'Vasuki' came he asked the purpose of the visitor with polite tone and ask him,
" What are you doing here Atithi(a guest), you are human, what work do you have with 'Nagas', and you clearly hiding your identity, I cannot feel any malice from you, so please tell us your perpose."
Karna' bowed to the king and said with polite tone, " Please forgive the intrusion Nag Raj, I came here with no malice and I cannot give away my identity so please Nag Raj don't ask for it, as perpose of coming , for a fellow unconscious human that intrude your kingdom, I came to ask permission to take him away."
'Vasuki' rises his eyebrows and question him," What reason for help to a unknown human?"
Karna' said, " we don't need any reason to help someone."
Vasuki was impressed with the answer he said ," you have no need to worry, he is a friend's son there will be no herm inflected upon him. We will neutralize his poison with a type of potion and thus this process give him tremendous ammount of strength."
He also said , " I am impressed by your, kindness, courage, and 'Dharm', so I would like to give you this potion."
Karna' join his hands and bowed to him and said, " Thank you, but I cannot accept this generosity, my parents never thought me to take something which is not belong to me or I earned myself."
'Vasuki' pleased with the answer and said, " I giving you a potion which will give you poison immunity, this is gift for looking out my friend's son. And you are a good human being,Please accept it. Don't deny it."
Karna' bowed to Nag Raj and Said, " Thank you for this gift. I 'Adhirath' putra 'Vasusena' and 'Radha' maa putra 'Radhey' Karna' will give you a promise that I will help you one time when you needed help from me."
Karna' also requested him, " Please don't disclose my identity to anyone."
'Vasuki' also smiled and said, " You are a very good person, you have moral that you cannot take something without giving something, I like your character. And I promise you that I Nag Raj 'Vasuki' will not disclose your identity to anyone."
Karna' smiled and said," In this world someone never gives anyone anything for free , except your mother and father."
'Vasuki' smiled and hugged Karna' and said," I cannot give you friendship of my 'Naga' clan but I can give you my friendship, my friend."
Karna' smiled and hugged back, " I accept your friendship my friend. "
Nag Raj 'Vasuki' recognised Bheem' as the son of Kunti' and Pandu'. The Nagas neutralised the poison in Bheem's body and by feeding him potion that increased his strength. Having befriended the Nagas , Bheem' returned home to his family.
Mahamantri 'Vidur' and Rajmata Kunti', tell everyone not to spread this news, this this lead to insult to Kurubanhsi.
All elders scolded 'Suryodhon' and nobody listen to his side, 'Suryodhon' return to Kauravs' with dispair, and his Mamasri 'Sukuni', his Mamasri concoled him and said,
" Everyone hates you 'Suryodhon', only by becoming king you can gain love and respect of everyone."
'Suryodhon' also said with sad voice but with determinatin, "While everyone is calling me 'Duryodhon' thin I will become 'Duryodhon' and become king and earn everything by myself."
'Sukuni' listening to this smile croockedly and rubbed his hand excitedly.
At night..
Karna' laying on bed and thinking today's incident , he is feeling their is something is going on, all things doesn't add up.
Why Bheem' is aggressive to Kauravs'?, why 'Suryodhon' is poisoning Bheem'?, Why everyone targeting Kauravs'?, Who is fuleing the flames?
He thought he should chenge the thinking perspective, who will gain something between Pandavs' and Kauravs'?,
The answer he got is only one person.
He who has hate whole Kurubanhsi, he who has lost everything by Kurubanhsi, he who vowed to watch dispair of Kurubanhsi, he who will use everyone in path of Revenge, he who will watch whole Kurubanhsi burned to the ground.
He is none other than Gandhar Raj 'Sukuni'.