Developed by Raven Software on the Quake 2 engine, Soldier of Fortune 1 was excessively gore and somewhat "realistic". They hired a real-life mercenary to help with the guns and gore, even though the main protagonist (named after the consultant) has the strange ability to rollerskate… and can also reload his shotgun faster than everyone's favorite mute scientist. The story kind of makes sense (a radical group trying to fund its organization by stealing nukes and reselling them - the bad guy, Deker, is of course German, a neo-nazi, and has some kind of armored suit a-la Metal Gear… go figure).
Soldier of Fortune: Double Helix cranked the gore even more, this time using the Quake III engine. The scenario is what you’d expect from a post-9/11 game: a bunch of mercenaries are enrolled to develop and distribute a highly-contagious deadly virus, while also developing a computer virus that would erase all news and information related to the biological virus (icing on the cake, those virus are named Romulus and Remus, because why the fuck not). Where the first episode merely had some hints of sci-fi, this new campaign devolves into the cheesiest B-Movie I've ever seen (with minor characters you don’t care about are being killed for no apparent reason and your boss is actually a mole). Levels are repetitive, music is annoying, the gameplay feels somewhat slower. While the game went for a more realistic feel, textures got blend.
Soldier of Fortune: Payback is… an incomprehensible mess of a budget game developed by Cauldron HQ and powered by their home-made engine, which is also an incomprehensible mess. The game is tedious at best, and features the worst gore effects to date, where every enemy is getting spontaneously dismembered. I only completed this game once, but I remember an incredible scene in a parking lot where you have to dodge cars as if you were playing a rail shooter. Worth trying out for a laugh.