I find it kind of funny how every time there's a sale of any type on any Steam game, you can always count on someone showing-up who bought the game recently acting like it's some personal affront against them specifically. News Flash: Sales happen (especially on weekends, mid-week, and on holidays). Neither Tripwire nor Valve is out to get you.
See, here's the thing: You bought the game, thus you agreed with it. You can't complain when it goes on sale later because you've already paid the price you were okay with paying. Sure, we'd all love to pay a little less, but we can't always get that. If you are looking at something you want to buy, you have to decide: Do I want this now at a higher price, or should I wait a while and see if the price goes down a little? This is especially true on Steam where any game can go on sale at any time.
If buying Killing Floor for only $5 was such a big deal, then you shouldn't have bought it when it was $6.60. Not saying that you should have had the clairvoyance to know a sale was coming soon, just saying that if $5 was the price you wanted, you should have waited until the game eventually (maybe) got put on sale for that price. Heck, that's how I do some of my shopping on Steam. I look at a game, decide "Sure, I want that, but cheaper". So I wishlist it and bide my time. Low and behold Christmas comes and I got about 3 or 4 games at the price I was hoping for, or lower.