In Splinter Cell Chaos Theory, most of the guards are just, well, guards doing their jobs. They may end up in the wrong organization, but they're not guilty of anything other than shooting at me when I'm an intruder. But in Conviction, they want each kill to be satisfying and you to have no remorse over them. They gave the guards annoying voice actors who swear and taunt you frequently, made them direct perpetrators in heinous acts such as detonating massive EMPs in Washington D.C, and having their conversations outside of combat being mostly about how they're gonna kill Sam Fisher and them being not repentant at all about the aforementioned heinous acts.