Why was John Keats called a poet “who was kill’d off by one critique”?

Question

In Canto XI of Lord Byron’s magnificent work Don Juan, romantic poet Keats is mentioned as a poet

who was kill’d off by one critique.

Why he was referred to like that? And which critique was it?

Answer 1

Shelley wrote, in his preface to Adonais: An Elegy on the Death of John Keats, that the critique directed against Keats's poem Endymion was enough to bring about his sudden death at the age of 25:

The genius of the lamented person to whose memory I have dedicated these unworthy verses, was not less delicate and fragile than it was beautiful; and where cankerworms abound, what wonder, if it's young flower was blighted in the bud? The savage criticism on his Endymion, which appeared in the Quarterly Review, produced the most violent effect on his susceptible mind; the agitation thus originated ended in the rupture of a blood-vessel in the lungs; a rapid consumption ensued, and the succeeding acknowledgments from more candid critics, of the true greatness of his powers, were ineffectual to heal the wound thus wantonly inflicted.

It may be well said, that these wretched men know not what they do. They scatter their insults and their slanders without heed as to whether the poisoned shaft lights on a heart made callous by many blows, or one, like Keats's composed of more penetrable stuff.

I found reference to this in George M. Rosa, "Stendhal et Keats", Romanische Forschungen 97(1) (1985), pp. 47-49, a journal article in French, which also says:

Byron, qui avait été averti de la perte de Keats par Shelley lui-même, mit en vers le jugement de celui-ci sur la cause de cette perte dans l'onzieme chant de Don Juan (chant publié en août 1823)

Byron, who had been informed of Keats's death by Shelley himself, set in verse the latter's judgment on the cause of said death in the eleventh canto of Don Juan (canto published in August 1823)

(translation mine)