Greta Thunberg Just Doesn't Know What 'Put Up Against The Wall' Means

It's silly to pretend that she was advocating killing non-climate-zealots.
My guess is that she just hangs around a lot of loony lefties, and has heard the phrase, but didn't realize it means to execute. Sounds like she means something more like to hold them to account or to make them do their jobs or something.
There's plenty of crazy on the left without making stuff up.