How was Anakin a Force Ghost at the end of "Return of the Jedi"?

I understand he saved Luke, but how was that him “turning” back to the Light side? He was the Dark side aaaallll the way up to that point, doing horrendous things.

That one act of saving his son and killing Palpatine was enough to convert him to good?

I find that odd. I am not sure that was “good” enough to redeem him.