As a quick recap, if you're unfamiliar, Resident Evil (or Biohazard as its known overseas, and possibly a more suitable title) initially involved a zombie outbreak in Raccoon City (to simplify things).
Of course, with the march of technology came improved or changed mechanics. Resident Evil 4, starring Raccoon City survivor Leon Kennedy, had a fixed camera angle, with over-the-shoulder aiming, which made it much easier to keep an eye on enemies and hit them properly, but also took the focus away from sheer horror and traditional zombies, making for a more action-movie like experience. It wasn't really so much scary anymore as it was tense. Ammo was still scarce. The monsters were tougher than your run-of-the-mill zombie. There were those bastards with the chainsaws. The improved mechanics made the game a lot smoother and more enjoyable to play, and the seeds were sown of modern Resident Evil.
Resident Evil 5, which I often like to use as a literary punching bag, took the series as far from horror as I've yet seen. Chris Redfield, another Raccoon City survivor and from here on out referred to as Punch Rockgroin, was the focus of RE5, a soldier investigating a possible bio-weapons outbreak in Africa. This entry in the series didn't even come close to horror, being an outright action film of a game with tentacle monsters and not a traditional zombie in sight. The mechanics in the game felt like a natural evolution, with excellent controls, tight aiming, weapons with great kick, and outstanding graphics and sound, but the inventory system was dire, the storyline unintelligible, and as I mentioned before, it was a big dumb action film. Where Punch Rockgroin literally punches a boulder at one point.
A side-title came out between these called Resident Evil: Operation Raccoon City. It was a pure action game that starred two groups of special operations groups, one in the employ of the US Government, and one in the employ of pharmaceutical and terminally suicidal/genocidal corporation Umbrella (originally responsible for the Raccoon City outbreak). It was a run-and-gun co-op game, that revolved around the events of the first game, with cameos by the first game's characters.
The more games come out in the series, the more the fans cry about how they're not horror. On the other hand, the more games come out in the series, the more money they make, with each one turning a profit easily. The films, which are more or less unrelated to the games, get bigger and dumber and louder with each entry, and make progressively more and more money each time. I like to posit the idea that Resident Evil doesn't necessarily have to be scary anymore, as it hasn't for quite some time. Change is, sometimes, good, and the series has changed into extraordinary people being total badasses fighting crazy monsters. Leave horror to the Silent Hills and Amnesias. I'll probably get crucified amongst gamers for this, but I think I kind of like Resident Evil how it is now.