Actually this doesn't indicate any change in design direction: Perytons have ALWAYS been able to do this. It's a core part of their fluff that they obsess over gaining "The perfect shadow" and so mercilessly attack (And kill) downed opponents. I'm 100% certain that whoever designed the Peryton, just like I did when I made mine for my own campaign had their Monsters of Faerun (or whatever) next to them. The 3E Peryton also had a similar named ability that let them tear an opponents heart out and kill them.Now if that doesn't signify a change in design direction, nothing does.
So being very "coup de grace" happy is sort of a thing that the Peryton does and I am absolutely delighted to see them have it. They're utterly, reprehensible and evil creatures, the very definition of the monster the DM is told WOULD coup-de grace in the description. Feast is a mechanic that keeps that feeling. In my own games, I gave Perytons a power called Heart Rending Bite, that only targets dying, prone or bloodied creatures. It is a minor action attack (1/turn) and is *specifically* used to kill dying PCs. It meant my PCs become terrified of these monsters very presence - you go down you have a good chance of staying down!
So this isn't so much a design direction as it is keeping with years of lore and other mechanics as to how this monster works. I've said it before and I'll say it again: It's not enough to have pretty fluff about how horrible Perytons are as monsters, their mechanics MUST back that fluff up to mean a thing. This is exactly what feast does.