He was the boyfriend of Annalise Keating for a while and had an affair with her. He contributed to the murder of Sam Keating by sending Rebecca Sutter to search for proof that Sam killed Lila Stangard. Nate murdered Ronald Miller

due to his suspicions that he ordered the hit on his father, Nathaniel Lahey, Sr..

Who Killed DA Miller?

Annalise's plan includes manipulating Gabriel into keeping quiet to protect his mother Vivian. Gabriel confronts the Keating Four about Sam and tells them that he's Sam's son. Later, the rest of the Keating Four and Oliver find out that Nate killed Miller.

Did Nate really destroy the files?

This is because information that was contained in the Denver files, which are seemingly only accessible to Nate, were included in the discovery file given to Annalise in season 6 episode 11. Nate told Annalise that he destroyed this file but his destroying it was never shown to the audience.