Horcruxes.
Voldemort didn't kill him, nor did anybody else.
Voldemort, Deatheaters, and in a few of the books Dementors
no. he almost dies, but then pretends to be dead while Voldemort tortures him and eventually kills Voldemort.
no
He decides to let Voldemort kill him, but all Voldemort did was kill a part of himself. Neville killed Nagini and Harry killed Voldemort with Expelliarmus
Yes, he does kill Lord Voldemort in the end of Deathly Hallows.
Lord Voldemort didn't actually know or care who Cedric Diggory was. Cedric ended up in the graveyard by mistake and Voldemort didn't want to leave any loose ends, so ordered Wormtail to "kill the spare".
Voldemort didn't just want to kill Muggle-borns in the second book. He had always wanted to kill them. The reason is because his father, who was a Muggle-born, left Voldemort's mother pregnant. Voldemort's mother was a witch, but after falling in love with Voldemort's father, she gave up all her powers. After that, when Voldemort realised that he was a wizard, he looked down to Muggle-borns.
Voldemort used the Killing Curse: 'Avada Kedavra'
Voldemort's own spell kills himself.
Nope, he doesn't
yes