Richard's competence and military ability was a threat to the throne and the legitimate heir Edward V. After a series of skirmishes with the forces of the widowed queen, anxious to restore her influence in the north, Richard had the young prince of Wales placed in the Tower. He was never seen again though his uncle kept up the pretence that Edward would be safely guarded until his upcoming coronation. The queen herself took sanctuary in Westminster Abbey, but Richard had her brother and father killed.
Edward's coronation was set for June, 1483. Richard planned his coup. First he divided the ruling Council, convincing his own followers of the need to have Lord Hastings executed for treason. (It had been Hastings who had informed him of the late King's death and the ambitions of the Queen's party). He then had his other young nephew Richard join Edward in the Tower. One day after that set for Edward's coronation, Richard was able to pressure the assembled Lords and Commons in Parliament to petition him to assume the kingship. After his immediate acceptance, he then rode to Westminster and was duly crowned as Richard III. His rivals had been defeated and the prospects for a long, stable reign looked promising. Then it all unraveled for the treacherous King.
It is one thing to kill a rival in battle but it is another matter to have your brother's children put to death. By being suspected of this evil deed, Richard condemned himself. Though the new king busied himself granting amnesty and largesse to all and sundry, he could never cleanse himself of the suspicion surrounding the murder of the young princes. He had his own son Edward invested as Prince of Wales, and thus heir to his throne, but revulsion soon set in to destroy what, for all intents and purposes, could have been a well-managed, competent royal administration.
It didn't help Richard much that even before he took the throne he had denounced the Queen "and her blood adherents," impugned the legitimacy of his own brother and his young nephews and stigmatized Henry Tudor's royal blood as bastard. The rebellion against him started with the defection of the Duke of Buckingham whose open support of the Lancastrian claimant overseas, Henry Tudor, transformed a situation which had previously favored Richard.
The king was defeated and killed at Bosworth Field in 1485, a battle that was as momentous for the future of England as had been Hastings in 1066. The battle ended the Wars of the Roses, and for all intents and purposes, the victory of Henry Tudor and his accession to the throne conveniently marks the end of the medieval and the beginning of England's modern period.
Sources:
1. www.britannia.com\history
2. www.numizmat.net
3. http://reference.allrefer.com/encyclopedia/Y/York-hou.html
4. http://www.britannia.com/history/monarchs
5. www.hotbot.com
6. www.yahoo.com