The one thing that kinda bothered me about RE4. It wasn't scary. But the game was still epic.
As for this matter. If Killzone 2 was released 2 weeks after RE5 than maybe it'll be affected. But because of the head start, people probably won't be able to resist getting Killzone 2 and go for it. If anything, I'm thinking RE5(PS3) sales will be affected by Killzone 2. But then again who knows. Most people should be able to get enough money for the 2 games if they really wanted it.
Being 2 totally different game is another. With WaW and R2. Both were pretty much bought for the multiplayer. Most people aint going to be playing 2 at the same time. Unless you bought it for the single player than ok.
You can finish RE5 and go to Killzone's MP without a prob. So I don't see too much prob. In the long run, sales won't really be affected. Word of mouth will keep the legs going.
