Simply not a good game. Plagued by stupid design that was popular at the time - namely monster closets - and bad guns that just didn't work well even if the shooting mechanics would have, which they also didn't. The entire last level is an infinite monster closet for example. It's worse than that, though, because instead of spawning off-screen in an actual corner or closet, Helghast legitimately spawn right on top of you in plain sight in the middle of a room. The one unique feature Killzone 1 had was also abandoned for the rest of the franchise - getting to choose one of four protagonists with different loadouts and combat styles. A lot of the guns had cool designs that reminded me a lot of Battletech TTG or Mechwarrior RPG, vaguely realistic but scifi at the same time, but they just didn't end up looking so cool on screen, either.