He failed in his plan to kill the train full of cops coming to Glenrowan, so he did one thing wrong
Nobody's perfect.
I still will never forgive how people watered down Ned's ideals to 'muh cops' instead of 'actual socialist revolutionary'.
EDIT: To clarify, Ned most likely didn't have any education in Marxist philosophy or theory. He was also born before the likes of the Eureka rebellion (problematic as it was), or any real socialist thought in Australia. However, many of his letters clearly laid out what he was fighting for (against the rich and their police lapdogs) and his actions showed followed through on that (for the most part).
wait he was an actual socialist and not just a guy who hated cops
FUCKING LIBS GOT TO ME, DAMMIT
That's what I thought it was. Okay for a moment I thought that Ned Kelley was running around quoting Marx but it makes sense that he didn't
