What is this wasp killing insect?
Given the large eyes, the almost non-existent antennae, the humped back, elongated abdomen and the wings, I'd say it is a robber fly. 



It is one of many insects known to prey on wasps.

Note the description on the linked page:


  This spindly piece of nastiness is a Robber Fly in the genus Diogmites. It seems that it's members of this particular genus that are adorned with the name Hanging Thief. You may remember that this was to denote their habit of dangling from a leg or two while the other limbs held onto prey, stabbed it to death with venom and then sucked out the insides. [Emphasis mine.]