He absolutely was for several reasons, many of which I cover on the Star Wars Wavelength in The Case For The Star Wars Prequels: Why They Don’t Deserve The Bad Rap (the link is in my bio above):

1)That’s how the Sith rules of succession worked: every Sith Lord since the time of Darth Bane got there by killing their master, and it fit with tradition that Vader would do the same.

2)He hated the fact that Palpatine got him to wipe out the Jedi, throw away everything, and left him stuck in that life support suit. He always resented the Emperor, but knew he couldn’t go at him directly because the Sith lightning would short his life support system out.

3)Until the moment he redeemed himself, Vader planned to off the Emperor and take his place as the leader of the Sith, with his son Luke as his apprentice. Since Luke didn’t rely on life support, he could use him as a proxy to destroy the Emperor and get rid of him that way.

Vader had planned to kill Palpatine almost from the moment he found out what he was, but the reasons he had for most of his life were vastly different from the reasons he had when he finally did the deed.