> ''Did you really believe them when they told you 'The Cause'? ''

A trick question, sadly. If yes, he's a sheep. If no, he's either a sheep or unprincipled or didn't have the guts to go to prison.

There's also an assumption that whatever "Cause" "they" told him about   was obviously a lie, and that now, being dead, he'd see it that way. Are we sure?

It's easy to get dead people to agree with us in poems. It happens in "Willie McBride's Reply" as well (also in "Flanders Fields"), just from a different point of view.