The index is weighted to the number of wars fought, the number of deaths without regard to which war is justified or not, jailed persons, crime committed, and so on. It is an attempt, but like any index, is bias. But the biggest mistake is to say this index is about anti war.
The only true value in this is not where the country is listed in which year, but how the ranks changed over time. For instance, the United States went from 97 in 2007 to 85 in 2010 while Cuba went from 52 to 77 from 2007 to 2010. So, is the US getting better or Cuba getting worse?