I'm so pissed off about that scene. It's done in the most disrespectful way possible, not only does it make everything about the story we know meaningless, it's also written in the worst way you could possibly imagine.

Immediately after the events of T2 where Sarah and John saves the world... When this movie starts they are at a beach and out of nowhere comes a terminator and just shoots John dead in a matter of seconds like he never even mattered. They make it so obvious that they were in a hurry to get rid of him, so much that they didn't even consider writing this with any thought, it's just "have someone show up and kill him, over and done with"

Ugh it's so insulting.