Picking the lesser of two evils? RE: the ending (MAJOR SPOILERS)

In the movie's finale, Light is in a hospital bed, speaking to his father.

"It's like you said: sometimes you gotta choose the lesser of two evils." (Light says)

"Which one are you, son?" replies Light's father.

No answer from Light.

I didn't understand what this meant. How did Light's father pick the lesser of two evils? What are the two evils that he has to pick between?

Please explain. Thanks in advance.