I've never played the Killzone games, owing to the fact that I don't possess the precise living room box required, but now at least I've gotten a taste of the Helghast. Killzone Source doesn't recreate an entire Killzone game in HL2, but provides a particular mission called Strange Company.
It represents several years of work by Moddb member zombiegames, and while I can't personally vouch for its representation of Killzone, it's pretty cool and looks great. With an AI companion, you fight your way through the interiors of a multi-floor building, battling Helghast at every turn, then proceed outside for more gunplay. You can carry a pistol and one of several rifles, use frag grenades, and are also armed with a knife. The Helghast look great, and I don't know if this is true of the original game, but they scream entertainingly when they die. Every time. I couldn't get enough of it.
They're tough as hell, too. Even on normal difficulty they killed me repeatedly. I had to disable AI using the Source console just to get close enough to them to take their pictures. At one point a dropship appeared over a shattered courtyard and they rappelled down on ropes in front of me. They're also pretty good at using grenades.
My AI companion was a little worthless in a fight, but she's still cool to hang out with. While I was remapping my keys (the default keybindings are a little odd) she helped herself to a soda from a vending machine.
The guns are fun to use, and plenty challenging due to recoil, and the maps are dressed with various bits of detail like Helghast propaganda. There are some other touches, like flying enemy drone that I presume has roots in the original game.
To play, you just need a Steam account and to have the opt-in beta of Source SDK Base 2013 Singleplayer installed. For the beta, right-click the SDK base on Steam, select properties, open the Beta tab and choose '-upcoming' from the list. (You don't need to input a beta access code.)
As for the mod itself, here's its page on Moddb. If you've tried it, and you have experience with the Killzone games, I'd love to hear if you think it's a faithful recreation of the mission.