How does the death penalty deter crime? 

From StatsCan (our national numbers folks):

Canadian Murder Rate Stays Low Without Capital Punishment

Murder Rates in Canada

The removal of capital punishment from the Canadian Criminal Code in 1976 has not led to an increase in the murder rate in Canada. In fact, Statistics Canada reports that the murder rate for 2003 was the lowest since 1967 at 1.73 murders for every 100,000 population.

The total number of murders in Canada in 2003 was 548, 34 fewer than in 2002.

Murder rates in Canada are generally about a third of those in the United States  (Washington DC is 80.6-ouch...)