* Vigilante groups become key weapon against Islamists
* Youth driven by popular anger against Boko Haram killings
* State backing for vigilantes risks trouble down the road
By Tim Cocks and Lanre Ola
ABUJA/MAIDUGURI, Aug 11 (Reuters) - Nigerian ironworker Ba Kaka initially felt sympathy for Boko Haram's violent uprising against a state he and many others saw as corrupt, un-Islamic and ...