I reject the implicit assumption that Zuckerberg has an ethical duty to reduce indirect harm from hosting content, and I also reject the implicit assumption that ethical calculations ("balancing" harm) are a meaningful way to reason about ethics. I think both of these assumptions are ethically problematic. The only reason Zuckerberg apologized to parents is because other people would have punished him had he not, which does not imply sound ethics or that he agrees with what he said.
I also think normalizing an infrastructure whose sole purpose is to suppress speech is ethically problematic. Rights are rarely expanded once taken.
You're right that people post abhorrent stuff on such sites, but I prefer approaches like filters over suppression. I also think that if reducing indirect harm is important to certain people (like reducing suicide), then there are known effective ways that don't require coercive power, like the public campaigns against smoking and MADD which have both had significant impacts.
Absolutely they should, and when I worked there that was known as "protecting voice", that content has always been explicitly allowed because it is free expression, even if reading it can be difficult for some people. The same with someone posting images of healed scars because they've been overcoming their self harm.
The content I'm talking about is graphic photos of suicide and self injury, fresh, blood soaked cuts, bodies hanging, graphic depictions of eating disorder (that goes beyond "thinspo", which is more borderline, and so downranked and not recommended rather than removed).
It's the latter that we believed (based on the advice of experts who we relied on for guidance) is harmful when consumed in large quantities.
