The messages on social media came after Adrián Hinojosa joined matadors for a lap of honour during a charity bullfight in Valencia to raise money for a children's cancer unit.
Adrián, who suffers from a rare type of bone cancer known as Ewing sarcoma and is undergoing chemotherapy treatment, said he dreamed of one day becoming a matador.
“This is a dream come true,” he said after being hoisted on the shoulders of a matador for a lap of honour waving a bullfighter's hat.
His father, Eduardo, told media that his son had been buoyed up by the experience. “He dreams of becoming a bullfighter, this experience has given him strength and raised his spirits as he battles cancer.”
But anti-bullfight campaigners were quick to express their disgust at the performance.
“What do I think? I'm not going to be politically correct. So. Just die, Just die already. A sick child who wants to get better so he that he can kill innocent and healthy herbivores who also want to live. Come onnnn! Adrián, you going to die,” wrote on a facebook account that has now been closed.
Another wrote: "What more needless spending is going on the treatment of Adrián, the child with cancer who wants to be a bullfighter and cut ears," wrote @manuel_ollero on twitter, referring to the prize awarded after a great bullfight.
The backlash over the comments has seen a flood of messages supporting Adrián, including groups that normally speak out against bullfighting and the hashtag #AdriánTeVasaCurar - Adrián you are going to be cured - was trending on Twitter in Spain.
Spain's national police also tweeted a message of support to Adrián. "Desiring the death of a child is despicable, cowardly and could be a crime. We are with you. You are going to win this battle!
The bullfighting community has leapt to publically support Adrián and denounce the ‘vile sentiments' against an ill child by those supposedly speaking up for animal rights.
With the agreement of Adrián's family, the Toro de Lidia Foundation, which takes on legal cases on behalf of the bullfighting community, said it would file a complaint against those responsible for the offensive posts.
A similar outrage occurred in the wake of the fatal goring of bullfighter Victor Barrio in July when animal rights activists gloated over his death in the ring.
A message from his mother read: "We only ask for respect for a child who is going through bad times and for his family too who only want to help realize his dreams."