Why so violent? Yes. It's a good question. It's a fight for control of a very lucrative business. The U.S. consumer demand drives the market, but it's the control of the supply where the violence occurs. It's an escalating thing. The beheading of rivals doesn't scare off the competition? Well, let's tear out their hearts and see if that does. It's a brutal, simplistic approach, but I think it is common in a gangster society, and especially one where the authorities are either cowed or corrupted.
Here is a 2009 article out of Harvard that explains the increase in violence since the rigid control of the PRI was removed. http://www.gov.harvard.edu/files/Sab...edOnline_0.pdf
This article http://www.bbc.co.uk/news/world-latin-america-10681249
has nice maps. At the bottom there is a map that lets you put your cursor over the particular state and see the death rate. There is a place at the top of the map to let you choose the year. There's also a chart at the very bottom. Chihuahua is particularly horrific, but is better in 2011 than 2010. In Yucatan the rate remains quite low.
This is an interesting blog with a recent entry on an arrest of one of the drug kingpins in late September and what that means. http://borderviolenceanalysis.typepad.com