This is what's I've been saying a billion times over:

I began in an interview of Guerrilla Games from OPM...UK? I think?

Basically, Guerrilla Games laid all their plans with Killzone on the table, and what they wanted to do seemed impossible (and it basically was impossible, as shown by the technical issues rampant throughout Killzone. Many things weren't fully realized until Killzone 2). OPM was so impressed by what they discussed with Guerrilla that they dubbed the game the "Halo Killer", and even put the term in big letters on the front page of the magazine.

SO ITS ALL OPM's FAULT!

Ok, looks like we found the right combination of words to get to origins of this, at least along the lines of Killzone.  Looks like it is one mix Playstation media hype, and one part Playstation fanboy, and then 5 parts XBox fans taunting.

I Googled:

OPM Killzone halo killer

And found this:

http://www.google.com/#hl=en&q=OPM+Killzone+halo+killer&btnG=Google+Search&aq=f&oq=OPM+Killzone+halo+killer&fp=Rhq7p-mUk-o

http://forums.metacritic.com/eve/forums/a/tpc/f/217108/m/3920040552

Im not sure if this rings a bell to anyone, but Guerrilla Games and SCEE have been making a FPS for the PS2 for some time now. It has been called the "Halo Killer" because it sorta resembles Halo, but it really isnt like Halo at all. It actually reminds you of Call of Duty combined with Rainbow Six, which works really well. The game is very deep, in total of 26 levels in all. There are about 30+ weapons in all. Also, there is 4 playable characters, each with their own story, and they also have their own skills with different guns and weapons. The game also has a very emotional and epic experience to it. If you are a FPS fan, this is one to definitly check out! I just wanna know what other people think about this game. And also this game is very, very gory!

http://www.1up.com/do/gameOverview?cId=2006481

1UP Killzone Preview
Killer of Halo or killed by the hype?

OPM Killzone Preview
After months of speculation, Sony's supposed "Halo-killer" is finally revealed.

And now onto Killzone 2 as "The Halo Killer"

http://www.n4g.com/xbox360/News-267041.aspx

"The Halo Killer"

The hype for Killzone 2 has been building for four years now. When the now infamous target render was shown at E3 in 2005 a backlash was created that still hurts Sony to this very day. Now with a month left until Killzone 2 is unleashed upon the masses GamingFront.net takes a look at how it has come to exceed every expectation that anyone could of had. Killzone 2 is the real deal. The real 'Halo Killer'.

So why is this game a Halo killer? There are three main reasons for it.

1. The single player is an epic sci-fi story.
2. The Multi-player is better than Halo.
3. Microsoft will suck the life out the Halo Franchise

Take the jump to read the full in-depth article.

There is nothing new here. Years ago, when Grand Theft Auto 3 hit big, all the birdmen began putting out Grand Theft Auto 3 clones. Years before that, it was first person shooters. More years before that, it was bloody fighters. One can find the birdmen back in the 8-bit generation making platformers. They would look at Super Mario Brothers and go, “Oh, I get it! We just need to make a game with cute music, colorful world, and upgrades like the magic mushroom!” Slapping wings on their arms, these games flopped. Amazingly, despite how many times the birdmen fall down, each generation they are ready to put on feathers and jump off a cliff.

After Halo 2 (and somewhat before), FPS would come out and try to cash in on the FPS trend happening in gaming. Somewhere down the line, the term was adopted to say that this game would be bigger then Halo. With a flood of FPSs on the consoles, the term refered to all of them.

It might have come from Killzone, Sony's answer to Halo.

i think it was halo killer. call of duty is the leader in fps now. so it should be call of duty killer, which is impossible in my eyes.

Halo Killer is what overly optimistic Sony fans call mediocre shooters.

This.  You know you've arrived when they start making new flavors of mountain dew in your honor.

it would need to beat halos sales on the 360 alone.... its close, it beats gears, but not halo yet.... we will see how far cod4 goes and cod waw... but id be surprised if they make it past 9 mill on the xbox, let alone where ever halo 3 ends in sales.... it will be interesting to see whats done for halo 4 ...

still its nothing compared to mario... and the hype for the greatest game ever super mario brothers 3.... how many video games get their own movie to push the video game