There indeed was a beginning. It began with the reanimation of Gordan Freeman to which he is told that he is once again needed for his expertise (or something like that). Of course, it would make sense for everything to be confusing. You were just in stasis for a long time, and suddenly are thrown into a future and unreckonizable world. Wouldn't that make sense?
There was a backstory. During the time of the invasion of the aliens, a new martian group appeared and seized control of the planet. Kinda like the Nazis in World War II or in any occupation situation. The reason for your mass murdering is simply to overthrow the regime alongside rebels (this is the backstory, and in no way shape or form a spoiler).
Sounds like reason enough to kill people. Why would you let some random group of whatever they are take over what is reightfully yours?
You know this backstory how exactly? From Episode 1 I'm betting.
The point is, Gordon Freeman, the character, the person who you are supposed to be playing doesn't know any of this stuff. So why is he doing it? It's not his town, it's not his life, he's only been there an hour or so before he starts shooting people. Some cute chick asks him to go from A to B so he goes to B and takes out 100 or so guys along the way? All I'm saying is that the story didn't draw me in, at all. I didn't give a damn about the story until Episode 1 (ie when they explain it to you in plain english). And not only that, okay, HL1 he was at the experiment, he knows what's going on so he does it no questions asked. HL2 Gordon knows jack ****, the world's gone to hell, wouldn't he open his mouth to ask "hey, what the hell happened here?"
It doesn't help when some of the in-game assets are crap and take you out of the moment. Like, why aren't there any girl zombies? And why aren't the zombies dressed like the civilians are dressed? All the zombies dress like Black Mesa scientists. How hard is it to do a texture re-skin so that the clothes of the corpses match the new environment of the game. Most of these faults are unfortunate btw, because the in-game acting by the NPCs in that game is the best I've seen thus far. At least they got their act together for the episodes.
Anyway. I should contribute to this topic but I don't have any candidates myself. Most endings don't blow me away one way or the other . . .
Oh. I will say though, not to get stuck on HL but the the climatic battle from Half Life 2 Episode 2 was pretty awesome. The end of the episode, whoop-de-doo an okay cliffhanger, but the end battle was great.
After the battle I'm walking into the base and people are like "good job freeman!" "thanks a lot freeman" and I'm saying to myself with a big grin on my face, "****ing rights I did a good job, WOOOOOO!"
FEAR:XP's ending was pretty good too, XP had much more atmosphere than the original game. Though that game is fundamentally flawed when it comes to trying to make you scared. I'm not going to be scared if I'm a super soldier fighting a bunch of normal punks.