Depends if they understood what they're doing. Many of them would have believed the propaganda whilst others would have realised they are being ruthless murderers.
Well that's a bit simplistic. For a start, in WWI propaganda and being 'ruthless murderers' has very little to do with it. It wasn't a war fought on propaganda, it was very much symptomatic of the national egoism which had been building since the nineteenth century - both sides were equally guilty of prolonging the slaughter. Also, with conscription, you didn't have much choice. At least in Britain, the strength of national feeling and the stigma attached to being a conscientious objector was so great as to make it fairly unusual.
As for the 'ruthless murderers', by which I presume you mean the deathshead squads of WWII - that's a bit different. The human psyche is a difficult thing to unravel, and not everything is black or white.