[cheebacheeba]

Don't allow yourself to enter into any such agreement.
Just think about it, if he held you to it, it would suck.
I think the point of discussion should be his ACTUAL reasons
for not wanting you to see these films, I mean, is it the violence, the language, the gore, all of the above...what?
And then, you need to find out WHAT it is about those aspects that he thinks will affect you in a negative way.
When you know what his actual problem IS, well, then you have a target to work upon. If horror has YET to affect you in a negative way, he doesn't really have that much of a leg to stand on, so you should just tell him that you only want to "broaden your horizons", and that his personal disliking of the genre shouldn't have to limit your life experience.

Just ask him if he SERIOUSLY thinks anything bad could come of it, I think you'll find his main argument will come down to
*He doesn't like it himself, and/or missed out as a result of similar actions by HIS parents, and how he turned out "just fine"
*The supposed "influence" factor.

If it's the first, tell him that times have changed, and in a world so fraught with horrors of it's own, in which day upon day you hear of the attrocities commited by your fellow man, you feel that the escape from all this that a horror MOVIE provides allows you to forget about all this for a while, even if only until the next time you see the news.

Or, if it's the latter, simply look at him and ask if he really considers you the type of person to go out and re-inact ANY acts from a horror movie/game...be sure to throw in the guilt trip, and state clearly about how ABSOLUTELY STUPID you think the people who have done so are, and that the root of their problems is much much deeper than having taken in a few horror movies.

If all else fails, just be frank with him and say
*That there's not really THAT much he can do about it anyway.
*That you may have repression issues by the time you're 18, and may in fact be more prone to going on a killing spree upon your eventual witness of R18 movies, and that you're worried, because you never know, you might start said killing spree AT HOME....