Standard SI prefixes apply. Thus Harold Shipman's achievements amount to 36 microhitlers. 

 The true utility of the hitler as an SI unit is it allows useful unit conversions. 

 For example: the EPA currently values a human life as being worth 6.9 million us dollars (6.9 megadollars). A simple unit conversion thus gives us 1 hitler is equivalent to -41,400,000,000,000 dollars. (-41 teradollars). 

 It can therefore be quantitatively established whether or not someone is "worse than hitler". When congress failed to pass a stimulus bill in 2008 the market lost 1.2 trillion dollars in 1 day, roughly equivalent to 29 millihitlers. Joseph Stalin is the only human I know of who can be called worse than hitler, as his achievements clocked roughly 5 hitlers. 

 When your bank nails you with a 35 dollar fine, you can confidently tell the teller that they are currently fucking you over to the tune of 84 picohitlers and ask if they have a very tiny auschwitz behind the counter.