I've never played half life, so I have no idea what it's all about.
It's about this guy going into work, just doing his job, but stuff goes haywire and he's the cause of hundreds of people's deaths - while he was just trying to do his job. Everywhere he goes people die, and he can never get a break Then it turns out he's some pawn in this other guy's plan, and he's put into a long-sleep like state for twenty years. He comes back and an elite race from another world has taken over Earth, and the main character is yet again, the indirect cause of thousands of people's deaths.
All because he went into work, did what he was told, and someone else forgeted up. Very tragic.