Neoitamus cyanurus is a Robber Fly (Asilidae family).
Robber Flies are also called Assassin Flies.
There are over 7000 described species of Robber Flies.
They are powerfully built, bristly flies with a short, stout proboscis (elongated mouthparts used for feeding).
The name "Robber Flies" reflects their notoriously aggressive predatory habits; they feed mainly or exclusively on other insects and as a rule they wait in ambush and catch their prey in flight. 
In general, Robber Flies attack a very wide range of prey, including other flies, beetles, butterflies and moths, various bees, ants, dragon and damselflies, ichneumon wasps, grasshoppers, and some spiders.
They do so apparently irrespective of any repugnatorial chemicals the prey may have at its disposal.
A Robber Fly attacks its prey by stabbing it with its short, strong proboscis injecting the victim with saliva containing neurotoxic and proteolytic enzymes which very rapidly paralyze the victim and soon digest the insides; the fly then sucks the liquefied material through the proboscis.

1st, 2nd & 3rd photo: A Neoitamus cyanurus feeding on a fly. In my yard, Hilversum, The Netherlands, 2 June 2017
4th photo: A male and female Neoitamus cyanurus mating. Laarder Wasmeer, Hilversum, The Netherlands, 16 June 2017