There are a couple indications it is more than a drunk driving fatality. There are newspaper articles within the apartment complex that mention a war going on, and specifically mention a bomb blast involving a bus. Additionally, as you get near the bus, there is a bunch of rubble, and collapsed buildings, but the protagonist muses that they were caused by the survivors, not the monsters. And in the green ending, the girl asks the protagonist to leave the city "when things get back to normal"; it's unlikely the zombie infection is real, and she is more likely referring to the fact that their city is involved in some sort of armed conflict. If it was "party + drunk driving accident", then literally none of those cues have any relevance to anything. The details of who the protagonist is and where he was at the time of the bus incident are vague, but it seems to be STRONGLY implied that the girl was on a bus that was caught in a bomb blast, and the protagonist was at the girl's side shortly before she died of her wounds.

• I think the red ending is the only one that implies the protagonist commits suicide or dies, since the protagonist and the blue man bleed when the blue man is shot.
• The blue (and red) ending implies that the protagonist lashes out at the illusionary world he's been living in instead of overcoming it; the blue man bleeds briefly when shot, then it's suddenly as if he was never shot at all, showing that the protagonist's efforts to overcome the delusion are in vain. He reunites with the girl (in a hollow sort of way) only because he has given up on escaping the fantasy/nightmare he has found himself in. This carries on the idea that the memory of the girl is imprisoned in his own mind, as you see earlier in the game when she is trapped behind bars. The general assumption with the red or blue ending is that the protagonist is confined to some sort of institution and is possibly heavily medicated and never recovers from the loss of the girl.
• In the green ending, he stops repressing that she is dead; the man wearing the cardboard box has some dialogue that implies the box is used to hide from something, and that something could be taken to mean reality or the truth, so the symbolism of taking the green pill is that he's admitting he has been hiding from the truth. The protagonist acknowledges the final moments he spent with the girl, and later is able to return to the spot on the hill where the girl was supposed to be after their hide and seek games. He knows she is gone, and is able to get back to living in the real world.