You could look at a few colonialists, like Cecil Rhodes for starters.
The shameful plotting of war for diamonds and gold, exploitation of local populaces almost amounts to slavery.
The British establishment in South Africa, and company owners would yield a long list of Britains worst.
The Irish are likely to name Oliver Cromwell, who started out with ideals, but changed dramatically over time, to deny things such as democratic principles he originally thought would be a better system of governance that Monarchical rule.
Harol Shipman, convicted of multiple counts of murder and who committed suicide in prison fairly recently, his estimated victim count ranges from 250 to over 500(his killings are so extensive that almost anyone who died whilst he was their medical practioner might have been killed outright or had their oncoming death hastened, as such, authorities can only make educated guesses).
His crimes are particulary terrible in that he killed using the trust patients have in their medical staff, that all his murders were extremely personal things, he had to directly and personally administer the fatal drugs, these are not the faceless spur of the moment or nasty orders given in warfare.
He knew all his victims.
He also stole from them jewellery and tried to alter their wills in his favour, and its the latter that brought him down. Stealing was not his principle motive, it would appear he just liked killing people on a one to one basis.
http://www.bbc.co.uk/crime/caseclosed/shipman.shtml
I think William the Conqueror would be a good candidate. His systematic use of terrorism to try draw his opponents into battle, and his subesquent clearing of districts in Norhtern Engalnd, aka ‘the harrying of the north’.
Its estimated that perhaps 100k died as a result of this one campaign, this was a campaign against any living person in those areas, not simply those capable of bearing arms.
His subsequent reign was bloody and ruthless, both in England and in France. The extent of his atrocities is very wide.