A declining murder rate is only one factor and probably a minor one at that when considering 'peace' around the world. Death tolls from wars and pestilence need to be considered first.
Needless to say, with WWI and WWII dominating the 1st half of it, the 20th Century has to be considered the bloodiest in world history, so far. Especially when you consider the tens of millions Mao allowed to starve in China in the 50's.
The 21st century is not off to a good start with political and economic unrest around the globe. Also, you have to consider we have over twice as many humans living on this earth than we did just 30 years ago. Too many babies lead to too many refugees. Too many bodies & souls that need nurture and most of all, resources like food and water that are getting scarce. As human density increases, so does crime and conflict.