To actually hold the PM accountable, it must be done either through the House of Commons acting as a court or the Hague Tribunal. Do you meerly wish to make him unemployed on his pension scheme. To defeat him at an election is easy but then he will take the option by convention that former PMs are elevated to the House of Lords as a Peer.
He will still be able to make more money than you or I ever will, but if tried and found guilty, he could be made to forfeit assests as a fine in conjunction to a term of imprisonment.
No monetary punishment can ever bring back those fallen, but could be used to aid those injured, and act as an example to others who undertake foolish and possibly illeagal actions.