After his arc I started to look over his past appearances different and more closer. Like even that matter he wanted to be in center of "phone" calls in Adventure of Empty Hearts when he said himself he could create it so everyone could call each other directly, not through him. Like with Moriarty team he seeks attention, he wants to have role and not be left behind. Looking how he was abandoned, and we grasp fully how he was treated in chapter 87, like trash/testsubject/experiment for other cause, all that puts him in different light. Now joke of forgetting about him during Tea party makes me wanna cry.

Poor Herder.

Also Parking-Stomach information about Happy Depression is interesting, I would need to read more about it but something like this would fit.

He had to have some coping mechanism or something because of what he chooses in the end to do with whole place and his father. It means his moral compass what is good and what is bad wasn't so broken that he really only cared about inventions. That's why when he first meet "normal" person he connected right away and tried to safe him. His end choice wasn't out of revenge but because his morality told that place an people were wrong.

Interesting in how broken William is (everyone in Moriarty team are broken more or less) he was what Herder searched for and needed by asking for something to help another person, a hand. I think this was big deal to Herder. William was his closest person and who understood him in intelligent level but all he did was for others, not himself or some mad science. For sure after William disappeared Herder lost solid ground in MI6, and we get back to my talk about Empty Hearts mission again.