Suddenly, Obi-Wan wasn't lying when he said Darth Vader murdered Luke's father. "From a certain point of view" isn't the equivocation of a manipulative old man. He isn't trying to trick Luke into killing his own father.
I think using words in a way that you
know will be misinterpreted to mean a very specific thing ("your father is physically dead and another person, Darth Vader, killed him") is pretty much lying. The only conclusion Luke could have drawn was untrue. That's a lie.
Using words that are intentionally misleading
in the context they are given is lying. Kenobi is lying to Luke, and his later "from a certain perspective" is just gaslighting. Luke should have answered, "Well obviously not from anyone's perspective except yours and his, Liar McLiarson."
If Kenobi didn't want to lie, it would have been easy enough to just say, "Your father was my best friend but after he turned to the Dark Side that part of him effectively died and he became Darth Vader." So he is clearly trying to deceive Luke.
Or, more likely, Lucas hadn't decided on that plot twist yet, or maybe had it in the back of his mind but didn't know there would be a sequel, and just kept the dialogue simple, leading to the awkward retcon later. Awkward retcons are a
Star Wars speciality - they made a whole movie to try to prove that Lucas didn't use the word "parsec" wrong in the first one.
So my read on the scene is that it wasn't originally written as a lie, but it awkwardly became one after
Empire was released.