If you ignored the original Killzone back in 2005 I can't blame you. It was a rough road for Guerilla as they tried to make an exclusive first-person shooter for Sony's underpowered console to out beat Halo and Metroid Prime. Sony never had an FPS that was exclusive to their system, so it was Guerilla's time to shine. It was dubbed "The Halo Killer" by fanboys and it sadly received lukewarm reviews upon release. I actually got this for Christmas 2005 and ate it up like candy and was a serious fanboy over this game. Fast forward 14 years later and I can see the game's many flaws, but also appreciate what it did for the PS2.
You play as a squad of four who are tasked with stopping an evil Nazi-like empire known as the Helghast that are trying to destroy all of humanity over the planet Vekta. The story is bare bones, but there's a lot of potential here with interesting art assets and great voice acting, but it just falls flat. There's no background on the main characters or the war you're fighting in. You're literally just dropped in with no background or reasoning behind it. This was never done with Halo or Metroid Prime as you were pulled right into the war or battles and understood exactly why you were there. It's just a bunch of cut-scenes with the squad going after various Generals and moving from Helghan base to Helghan base.
The shooting is very interesting as it's part of why Killzone was loved by those who did like it. There's a lot of weight behind the weapons and they are actually quite unique and shoot well. It's your standard array of military weapons but with a twist and with some personality behind them. Honestly, the weapons are the only thing front and center in Killzone and dominate the entire game. From the ISA and Helghast standard assault rifles to rocket launchers, grenade launchers, pistols, and heavy machine guns. Most weapons have a secondary fire that helps balance their weaknesses such as the Helghan's rifle has a shotgun attachment and the ISA one has a grenade launcher. I personally stuck with the Helghast assault rifle through the first fourth of the game as you can only play as Templar, but once the other characters were unlocked I played as Rico since he has a chain gun with 800 rounds and a missile launcher. It's seriously overpowered but feels so good to mow everyone down around you. The only weapon I really disliked was the shotgun as it's so slow to pump and shoot that unless you are one on one with a single enemy you'll get killed because you can't fire fast enough. It's practically useless even in tight corridors.
The downside to the weapons having a lot of weight behind them is the animations. They are so long that is makes the game more difficult than it needs to be. Every time you switch weapons there's a long animation of pumping the shotgun, fiddling with a rocket launcher scope, flipping up the lid on the scope to the sniper rifle, Rico's chain gun takes at least 2-3 seconds to swing out then there's a weird pump animation after so you have to hide behind walls every time you switch weapons because of these animations. Same goes for reloading. Some weapons take over 5 seconds to reload with the shotgun taking nearly 10 to load every single round. It's fun to see and was never done up to that point in time, but it needed to be sped up or changed.
Outside of the weapons the enemy AI is as dumb as a doorknob with the Helghast literally standing around not shooting at all or they won't move to cover. Part of this is the underpowered CPU in the PS2 and the game engine that pushed it way too far. There's so much pop-in with fog of war to make up for the lack of a draw distance that enemies pop in and most of the time all at once so an entire room or corner will be full of Helghasts that are easy pickings with a single grenade or kill you really quickly if you don't notice them. The game engine just chokes the PS2 like no other game with framerates dipping into single digits. I also played the remastered version for PS3 and while the 720p resolution looks sharper, there's still framerate drops because the engine just couldn't handle the load. Sadly, because of the limitations, environments are bland and boring with claustrophobic corridors and almost no draw distance. The game is gray, dull, and colorless, and while this could have worked, the weak PS2 brings the art backward because not enough can be rendered on the screen to make it look nice.
The entire game pretty much plays the same way and takes about 5 hours to finish. There are no vehicle scenes, no scripted events, it's all just running and gunning which gets old towards the end. For the PS2, this game is quite impressive and had a decent multiplayer mode, but there are more flaws than there are perfections. I would rather have had an uglier game that played better, but PS2 fanboys were clamoring for something that pushed the system like Halo did the Xbox. Is Killzone a Halo killer? Absolutely not and doesn't even come close.