He gives them orders.
They are both servants of Lord Voldemort. Why wouldn't they talk to each other?
Dumbledore convinced him there was no love in Voldemort, his death eaters were servants rather than friends and his only concern was himself.
Sirius Black was murdered by his cousin, Bellatrix Lestrange, because she is the crazy, murderous type. He wasn't on Voldemort's side and Bellatrix was one of Voldemort's loyal servants.
Because voldemort thinks that snape is the master of the elder wand but he isn't.
Answer: No. Voldemort was stronger. He was the strongest wizard known to wizard kind aside from Dumbledore. Answer: Most likely, but one couldn't tell for sure, because Slytherin lived a thousand years before Voldemort. (Note: Voldemort was very proud to be Slytherin's Heir.)
Snape was an accomplished Occlumens.
she didnt tell him because she didn't care about the war anymore. All she and her husband wanted was to find their son. They lost all favor with voldemort in the 5th book when they didn't get the prophecy
To test if he's become dark.
Hold the torch, basically a servants job.
Lily didn't tell James that Voldemort was coming, she told him that she was putting Harry to bed. Neither of them were aware that Peter had betrayed them.
I would say that Draco is good. Throughout the book you can tell he doesn't want to be doing anything for Voldemort but he was threatened into doing what Voldemort says.
He made Regulus (and his house-elf, Kreacher) help hiding the locket-horcrux in the cave.