Chilling new details have emerged in the execution of a Simi Valley doctor and his wife — their killer son sat in wait to ambush and gun down the beloved couple in their own garage, a neighbor told The Post.

Ketih Cordes drove past the home in his black Honda Civic, then staked it out as he waited for the perfect moment to strike, the neighbor said Wednesday.

“He did like a drive-by, like a dry run at 11:25 and then went and staked out at that cul de sac,” said Mike Hylton, who lives five houses down from the bloodied crime scene.

“Then at 12:01 he must have waited for the garage door to open and that was it,” the neighbor said.

That’s when Hylton’s son heard shots fired, looked outside and saw a car take off. Hylton ran to the couple’s home where he saw Dr. Eric Cordes, 63, and his

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