The "It's an old movie" excuse is shit. There is literally not enough time in the life span of a human to watch every movie ever made, so there will necessarily be old movies that people haven't seen yet, and may in the future. Although it's impossible for them to go through life without running into one spoiler, it's still understandable to get upset on the occasions that it does happen. And it's ridiculous to make up excuses for doing something that bothers someone, when the actions to prevent that come in at an almost zero cost to the person posting the spoiler. Mistakes happen and that's okay, but I can't really think of any way to justify it other than "Oops, sorry. I made a mistake. I didn't think it through." Any other justification is just asshole-ish.
Also, no. I'd like to think I wouldn't kill him. Obviously, it's pretty damn hard to imagine the situation, but I don't have much of a temper, so I doubt I'd murder the guy.