Generally, those who think that the world would be better off without religion seem to have made some vague claim, such as:
Religion has caused wars, acts of terrorism, murder, discrimination, and genocide.
or
Religion has caused more problems than it has provided solutions for.
Mycernius has listed some atrocities in the name of religion. No-one has made any serious attempt to prove this claim that religion does more harm than good. How about we start with casualties of war? This site
http://users.erols.com/mwhite28/warstatx.htm has very detailed breakdowns of the casualties in 20th century conflicts and some stats for earlier wars and genocides too.
To pick out a few of the usual suspects, here are some death tolls from primarily religious conflicts:
Rise of Islam - less than 700,000
The Crusades - at most 5,000,000
Witch hunts - realistic estimates no more than 100,000
Spanish Inquisition - at most 32,000 deaths, at most 341,000 punishments
English Civil War - at most 100,000, including deaths not directly from conflict
Holocaust (Jews) - 6,000,000
Pakistani independence - at most 1,000,000
Northern Ireland - 3,500
Jonestone - 913
Waco - 72
9/11 - approx 3,000
compare these to primarily non-religious wars and genocides:
WWI - 15,000,000
Russian Civil War - 9,000,000
Stalin's regime - 20,000,000
WWII - 49,000,000 (since we counted 6m Jews under religious wars)
Chinese Civil War - 2,500,000
Mao Zedong's regime - 40,000,000