explain what the author means by "imagination was a killer"

This metaphor directly relates to the soldiers' making things worse for themselves before starting, meaning that they'd have themselves tied up in knots before a task was even begun. In contact, the author is specifically talking about the destruction of tunnels, which was made even more difficult by the men imagining all of the terrible things that could happen before they even entered..... in other words..... their imaginations made them paranoid.