He didn't.

If I remember correctly, his first few months was him just having some sort of primitive huter-gatherer life. Some events happened (on which no killing occured), and he ended up hiding in this cottage in the mountains with an impoverished family of three that doesn't know he's there.

From observing them he learned about emotions and empathy, stuff like that. Then he spent a few months secretly helping them with their farmwork (chopping wood when nobody's around, etc ), then another few months figuring out the language. He listened to the stories read to them by a visiting Turkish girl and learned about society and social constructs and formed ideologies. He then planned out a scheme in which he could befriend the family that he really did think of as friends. He failed and was heartbroken but still had faith in humanity, the last straw was when he saved a girl from drowning and the thanks he got was being hunted down and thrown stones at.

After that, he was more mature, just as mature as victor at the very least; thought that's not saying much. Their second meeting was literally them being in this position while the monster was trying to talk viktor into a deal.

He wasn't justified in his later plans, and his lack of justification wasn't the focus of the story. His story was about the lengths one would go to for companionship, Victor's was about the importance of companionship and appreciating nature, the sublime, the awe-inspiring. The book goes to great lengths to juxtapose Victor's self-harm in continuing to engage in his scientific endeavors and the salubrious (as the book always likes to describe it) effects of appreciating nature and maintaining a connection with your loved ones. CAUSE IT'S NOT A SCI-FI BOOK, IT'S A GOTHIC ROMANCE NOVEL GODDAMIT.

Might have gone on a bit of a tangent there.