You're drawing the "this makes an awful person" line at a super arbitrary point.
peter singer believes that if you could save a child's life for two dollars, and you instead spend that money on a coffee, you're in effect killing the child. if you extrapolate this reasoning to it's logical conclusion, it follows that you should be living at the same level as the poorest person in the world so you can donate as much money as possible and still survive.
however, if you start living in a refrigerator box and not doing laundry because you don't want to spend money on detergent, you're likely to lose your cushy tech job for being weird, and then you'll be able to save way fewer children.
so, while it's true that you can go be a hermit in the mountains who only eats berries extracted from bear poop, that might not actually be the most effective way to help the environment. it would preclude you, for instance, from trying to convince someone on hacker news to live a more modest lifestyle, thereby doubling your contribution.