In Tartoros Arc of Fairy Tail, Gray was given Ice Devil Slayer magic. His father then said, "Do you know why I learned ice devil slayer magic? It's because END is a fire demon." Gray said "I swear one day I will kill END". Why and how did Natsu kill Gray's father when he was still living?
Natsu didn't kill Silver; in fact, nobody directly killed him. Silver was already technically dead for the entire span of the series starting from the time when Deliora destroyed Gray's home; he was just kept in a "living state" with Keyes' necromancy curse. Silver died (for real) because Juvia killed Keyes, causing the necromancer's power keeping Silver "alive" to disappear, which finally lead to Silver's death.
Silver Fullbuster was an Ice Devil Slayer, the father of Fairy Tail Mage Gray Fullbuster, and a member of Tartaros' Nine Demon Gates. He died in X774 along with his wife, Mika, but was then resurrected by Keyes, serving as a test subject for his experiments.
And Deliora is the Demon that kill Gray's Family, so Silver starting to kill all Demon he came accross.