ID this insect please. SE Brazil Oct 2017
This is, for sure, a robber fly (Diptera, Asilidae). My guess is that this one belongs to the Genus Promachus.

Here is a photo of Promachus rufipes (from North America) for comparison:



Coincidentally, even the prey (a wasp) is in the same position of the one in your photo!

This Genus has 9 species. If this is in fact Promachus, you'll have to find out which are found in South America.