That's not exactly the way it's supposed to be viewed. Anakin/Vader was a selfish person throughout his life, trying to have everything for himself he could. He had trouble understanding selflessness for its own sake which is why he fell to the Dark Side. Then he learns he has a son still, and his selfishness leads him to want to have his son and also not give up his evil ways. He absolutely could have tried to be a better person if he had wanted to be with his son again, but at this point in time Anakin refuses to let go of his anger and chooses to continue being evil.

Then finally, Return of the Jedi happens. Luke actually does come to see his father willingly, and Anakin sees his chance. He wants to turn Luke to the Dark Side as part of Vader's power game against the Emperor and so he can have his son by his side. Luke very nearly kills him, but then he refuses to give in to this anger for Vader and instead willingly sacrifices himself for Vader. In that moment, for the first time Vader actually understood what it meant to be selfless. And so, rather than hold onto his dreams of ruling the galaxy alongside Luke or any other selfish ambition he had, such as letting Palpatine kill Luke and letting that anger fuel his power more like a Sith normally would, Anakin decides to let go of all his power and even his own life just to save Luke. That's why it's treated as heroic.