I believe the fire bombing of Dresden was worse and caused more deaths than both the atomic bombs.

On the 13th February 1945, 773 Avro Lancasters bombed Dresden. During the next two days the USAAF sent over 527 heavy bombers to follow up the RAF attack. Dresden was nearly totally destroyed. As a result of the firestorm it was afterwards impossible to count the number of victims. Recent research suggest that 35,000 were killed but some German sources have argued that it was over 100,000.
Estimates of civilian casualties vary greatly, but recent publications place the figure between 24,000 and 40,000.
Mans inhumanity to man

Today's historians estimate a death toll of between 25,000 and 40,000, with an independent investigation commissioned by the city itself to be released some time in 2008.