>How can grown ass adults be so pathetically mean to a single person because he wrote (free & gratis!) software they don't like. Seriously, it's mystifying.

Because he and his orbiters were extraordinarily arrogant and condescending about their software. And let's not forget that while it's pretty good now, in the early days systemd was not very good at all. Because things like GNOME 3 decided to make systemd a hard dependency, pretty soon it became nearly impossible to avoid using the entire systemd stack even though it was nominally modular.

It's not just that he wrote $thing that someone didn't like. It's that $thing got shoved down everyone's throats at a time when $thing didn't work very well, and then when people rightfully objected, they were belittled by Poettering and his fan base.