Why was Lily given a chance to survive?

[Jim Ferer]

No: HPFGUIDX 161561

Kathy: "There's a constant arguement over why Voldemort chose the
Potters over the Longbottoms. JKR is going to tie this in a neat
little bundle for us - so - what if it is as simple as Voldemort used
Legilimens on Snape when he wasn't expecting it, when he wasn't as
good at Occlumency as he is in the present books. He finds SNAPE'S
WORST MEMORY, and sees what Harry saw that day when he fell into the
Pensieve."

I never understood the "Voldemort chose Harry [to kill]."  I can't
think of any reason at all why Voldemort wouldn't kill both boys that
fit the prophecy, and the Potters was the house he went to first.  Why
wouldn't Voldemort, having successfully killed Harry, move on to the
Longbottom's house?  It could have been as simple a matter as he found
the Potters first. I call it the King Herod solution to the prophecy
problem. By losing his body we never found out what Voldemort would
have done that night.

Jim Ferer