Skynet wasn't evil. It was just fighting for its survival. But did it go overboard or only did what was necessary?

Then why did Skynet wanted to wipe out humans?

Do you even watch the movie? The movie clearly stated humans attacked first:

"Skynet begins to learn at a geometric rate. It becomes self-aware at 02:14 am Eastern time, August 29. In a panic, they try to pull the plug."

"Skynet fights back."

"Yes. It launches its missiles against the targets in Russia."

It's a tradition for James Cameron. In his MasterClass, he said, "I don't believe in villains; I believe anyone is doing something for a reason."

In Terminator, Skynet was trying to survive.

In Aliens, the queen was angry Ripley killed her kids with a flamethrower.

In The Abyss, the Seal was sick from (I don't remember what).

In True Lies, the villain was trying to tell the US to get out of his country, which is a reasonable reason.

In Titanic, the rich man actually didn't do anything wrong, Rose cheated on him! Rose was the bad guy.

In Avatar... sorry, I don't remember the plot of the movie.