None of the sources before More doubt that Edward IV was solely responsible for the death of Clarence, even if they were in some doubt as to why he was executed. More hints, in very obscure language, (quoted by Gairdner in his Life of Richard III [1898, pp.34-35] with the remark that More was obviously not sure himself) that Richard of Gloucester may have encouraged Edward to execute his brother, but goes no further. He was not followed in this by any other chronicler, who mostly quote Vergil. Vergil in turn says more or less the same as Mancini. Shakespeare, in a major contribution to the legend, seems to be solely responsible for making Richard of Gloucester the prime mover in the death of Clarence and he was followed by some subsequent writers. Richard has however never been seriously regarded by historians as responsible for the death of his brother.
The reason for the execution of Clarence has been much debated. It is most unlikely to be because of the alleged prophecy that Edward would be succeeded by someone whose name began with a ‘G’ as said by Rous and Vergil and subsequent authors (see ‘The Prophecy of G’, Anne F. Sutton and Livia Visser-Fuchs, The Ricardian, vol. 8, no. 110,  pp.449-450). The official reason for his execution was that Clarence had committed treason by keeping an exemplification of the Act of the 1470-71 readeption Parliament making him the heir to the throne after Henry VI and his son. There is no eviidence that such an Act ever existed, the Roll of this Parliament has disappeared and Professor Lander argued that the charge was a fabrication (J.R. Lander, ‘Treason and Death of the Duke of Clarence: a Re-interpretation’ Canadian Journal of History, vol. 2,  pp.1-28), an argument disputed by Hicks (False, Fleeting, Perjur’d Clarence, pp. 159-169). Hicks discusses the Act of Attainder and other evidence and concludes that Edward IV probably had a mixture of reasons (mostly involving treasonable acts) for condemning his brother. Other recent work on Clarence is as a local magnate (Christine Carpenter, ‘The Duke of Clarence and the Midlands: a study in the interplay of local and national politics’, Midland History, vol. 11,  pp. 23-48, a theme further discussed in Carpenter’sLocality and polity: A study of Warwickshire and landed society, 1401-1499,  and as ‘good lord’ to his followers (M.A. Hicks, ‘Restraint, Mediation and Private Justice: George Duke of Clarence as ‘Good Lord’, J. Legal History,vol. 4,  pp.56-71). PWH