Funnily enough, I see the Monster route as being far more damaging. How can you grow to love others if you cannot even love yourself? He'll become the epitome of an edgy, socially irascible teenager until he snaps out of it and realizes that he does deserve to live.
The fate of those who trusted the Flamels is not his fault. With the Monster route, he is accusing himself of the crime of being born; how could you possibly think that is acceptable? A hump to get over? Are you serious? He's a thirteen-year-old boy, for crying out loud!
And on that matter, his "disregard for others" is just basic social awkwardness. He has clearly shown a capacity for love, an interest in the world--he wants to live. How many adults can say that? How many people here can remember clearly depressive episodes, of apathy and self-loathing? I know I can, and I wouldn't wish that kind of self-doubt on anybody.
You want to take that love from him so he can struggle all over again to get it back.
I can't fathom why.