The Siege of Leningrad, Battle of Berlin, and Battle of Stalingrad, all during WWII, are said to have had the biggest number of casualties.
The number of casualties for each of these have only been estimated, so it's hard to say what the bloodiest is.
As for bloody killing methods, I'm not sure statistics are kept on them.
Battle of Towton is said to be the "bloodiest battle ever fought on English soil" though.
