1. Jewish State of Israel in the Levant (JSIL) - with death rates of close to 75% civilians, it's not even close. There is no other organized military that can compete with their atrocities, when scaling for per-capita civilian deaths. They deliberately target women and children to "teach a lesson" to the population not to support their freedom fighters
2. Saudi Arabia - one of the most backward societies on earth, terrible human rights record
3. Islamic State - ed up dudes, but have only been at it a few years, otherwise they'd be higher
4. Ukraine - Ultra nationalist neo-nazis
5. US - admittedly the past 5 years or so have taken us out of the top 2, but extra-judicial drone murder in the hundreds across multiple countries while the obedient "free press" ignores every atrocity is quite disturbing