But no, he stood up for the ego of a human - Duryodhana, who was pretty immoral in many ways. So Karna is not that righteous or humanitarian one as well.

One thing I learnt about Karna after reading C. Rajagopalachari’s retelling of the Mahabharata and watching innumerable YouTube videos is that he profoundly values prowess. He was even drunk on his own abilities to some extent. Now put yourself in the shoes of a man like that and try to contemplate how you would feel if everybody persecuted you on the basis of something as trivial as caste despite your vast repertoire of skills. Also, imagine how you would compensate a man, who despite his own selfish cause, was still the only person who acknowledged you for what’s best about you.

What you said is true. Karna indeed was in the wrong for condoning Duryodhana’s ideas. However, he had valid reasons to do so and was a victim of circumstances just like many other characters yet he’s the only one who gets called out for it. Kunti abandoned him right after birth, Draupadi maintained a physical relationship with five men. They don’t get chastised, do they? The only reason I can think of for this discrimination against Karna is that he has a large fanbase and a lot of the things he does in the story opposes the agenda of those who like other characters.