(One of the best deaths I have seen)
@TheRedRose: The man in the purple jacket is Ryūnosuke Uryū, and he is a serial killer. He found a book at the house of his parents in which it's described how to summon demons (actually servants for the Holy Grail war, but as he is no magus and has no knowledge about it he just labeled them as demons) and then he wanted to find out if it's true.
He had already killed a few families and used their blood for the ritual but with no success. In that scene he already killed the family and left the kid as a offering for the "demon". And well he managed to summon a servant. That other guy is the servant he summoned and he is actually Gilles de Rais, who during his lifetime was a serial killer of children.
At first he lets the kid go and Uryū protests, but after the kid gets killed Gilles explains to him that he finds normal death boring and that it's about the death that happens after the felling of safety and flicker of hope because it's a more "dynamic" one . Because of that Uryū gets along with him as he enjoys killing in unique way and they abandon the Holy Grail war and just go on a random killing spree.