That is incorrect information. Karna was cursed because he lied, not because of anything else.
Karna was kind of 'kshatriya' (warrior). I say kind of because, even though he was born to Surya (sun-god), he was raised by a charioteer, which is what the world knew him as.
3 varnas - Brahmanas, Kshatriyas & Vaishyas - are supposed to study Vedas (whether they do it nowadays is a different matter), each one learning the parts needed for their duties later in life.
So, simply learning Vedas pertaining to their varna does not convert them into a different varna e.g. if a kshatriya learns DhanurVeda, he does not become into a brahmin.
Now, Parashurama had taken a vow that he would only teach warfare to Brahmins because he knew first-hand, how destructive weapons can be in the hands of an angry person. Brahmins had control of their senses and were calm and patient. So they would only use it for good of others, instead of gaining power.
When Karna went to Parashurama, he lied that he was Brahmin. When later Parashurama found out that he was not, he was cursed.
Karna already got teaching of all the Vedas before he goes to Parsurama so technically he was a brahman.
Where did you read that he learned Vedas before going to Parashurama? He went to Drona first and got rejected and then went to Parashurama.
Like I said before, even IF he had learned Vedas, that does not make him into a brahmin.
Most who consider that Parashuram cursed Karna are quite wrong in their understanding about Bhagwan Parshuram himself, Bhagwan represented the force who believed in equality, standing up for the weaker sections and using knowledge and force for the greater benefit of society. When Parashuram is questioned by Karna , this is what Bhagwan had to answer ”Any knowledge gained by lie or deciet is fruitless and will definitely fail to deliver when one need it the most, this is not my curse but law of nature”
When Karna does counter him saying the varna to which he belongs would have denied him the access to the knowledge and hence he had to lie, Bhagwan outrightly denies his claim saying “He or his lineage never believed in Varna system which is evident through fact that the followers of Jamadagni and Parashuram had been from all the caste, it is just due personal tragedy that happened to his family in past he vowed to tutor only Brahmins (Although he tutored Devvrat too, but as a Ganga son and a vasu) and that it was harmless personal choice he took. He also assures him that if at all he did tell truth and expressed his deep desire to learn he surely would have delegated him to the best teachers he knew and ensure that Karna is not deprived of the chance to learn. ”
Bhagwan was extremely sad with the incident that even after teaching so many years, he failed to teach his student to “stand up and speak truth” and thereby reducing himself to a mere instructor rather than being a teacher. This incident saddened him so much that, he never mostly did accept anyone as student then on and took it as failure to be a good teacher.
As explained by Krishna later, Karna was bound to loose the knowledge as his mind always revolving around his competition with Partha and hence the half hearted learning is bound to fail whether one is cursed or not.
HENCE IT Is A CURSE KARNA BROUGHT UPON ON HIMSELF, WHO SOUGHT EDUCATION AS MERE TOOL TO SHOWOFF AND GAIN HOLLOW GLORY RATHER THAN USE IT FOR THE UPLIFTMENT OF DEPRIVED AND BETTERMENT OF SOCIETY.
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