Could you check if the new aiming method is (still) on for you in the options? You may have experimented with it or not used the new system at all, but I remember having a similar experience as you did when I went back into the game and everyone was using the new system but me ... When I switched kills came a lot easier.
Of course, there are also percentagewise more seasoned players in Killzone 2 multiplayer now than before, so that could be it too. But the faster aiming that was introduced a while ago and made default for new users since is definitely very important.
Actually, since you have two PS3s and probably never played it on the one at home, that's probably it.
Arwin, you are right ! The KZ2 game on my home PS3 is a fresh new start. It does not have the "High Precision" checked. I was probably fighting with experienced gamers on the high precision settings last night.
EDIT: This may be a big problem for new, unsuspecting KZ2 players. GG should turn on "High Precision" by default if most of the players do that.