To add my grain of salt to what Gian-Reto and deltaKshatriya said, IMO, if you have to ask, then it is indeed "too" offensive, which, for me, is exactly the point of why you should make it, especially if it's an experiment.
I think video games are an amazing way to make experiments, and see how things turn out. For me, it's a medium that is made to push boundaries, and test the limits. And as a few mentionned, it wouldn't be the first time too offensive games were made, and were successful.
I don't think making a game on a subject is by default endorsing it. IMO, it depends on how you present it. It all depends on why you created it. I don't think RockStar and the others encourage violance in real life. Often, it's more a way to escape, and to experiment things that you wouldn't experiment in real life. And it's also often a way to point out quircks of today's society by exaggerating them.
However, it is indeed a touchy subject, and you should expect a lot of criticism, and heat rising from it. But if you're ready for it, and if you are at peace with why you created this game and why it is appropriate for you to have created it, then go for it!
Sure, but:
That is not how the OP framed his question or idea. If he made it CLEAR that his game would show violence against civilians and terrorism as a medium to reflect on what that violence does to people (both the aggressors and the victims), how it changes society for the worse, and/or how in the end it benefits no one besides some few sleazy guys making profits from selling weapons and outfitting armies and terrorists alike, I might have had different words to say.
This certainly is an important question to ponder. ESPECIALLY as in our society, terrorism has become a wrongly used term. As soon as someone declares war on terrorism, he is the good guy. No matter if that war on terrorism will mostly serve to oppress a minority that has done nothing wrong other than wanting to live on the land of their ancestors and keep their language and traditions.
Now, even if the OPs question would be framed like that, I still had my doubts if showing open violence, and letting the player loose in a sandbox is the best way to ask these question.
Giving a player a sandbox where he can run over people and kill prostitutes, and expecting the player to ask deep philosophical questions and make conclusions is like giving a tasmanian devil a piece of meat and expecting him to have deep thoughts about the circle of life and the nature of surviving by killing other animals while feeding on the meat...
That might be a rather negative opinion about GTA players, but lets face it: while the GTA campaign MAYBE quite clever and give you a lot of context and maybe one or two philosophical questions together with the mayhem and violence (don't know, never played GTA), the open sandbox where most people are "experimenting" is not really giving anything other than... an empty sandbox to do what ever you like.
Sure, most people are just doing experimenting with it, they just want to see what they can do to get a chuckle out of it, they might go cuddle a bunny if that would be something no other game has done before. Still, when you can run over civilians and do stuff like that WITHOUT context, or with a context framed wrong (opinion alert), your game is promoting violence.
Sure, running over people is what a criminal might do, so if you play the sandbox as a criminal, that is actually a realistic thing within that context. But getting points for running them over... erh. No. UNLESS the context is a "run this specific person over because <contrived story reason #1>" quest, and you are getting point only for killing that one person, your are rewarding players for violence without context, which to me (opinion alert) is highly wrong.
TL;DR: an open sandbox is actually one of the games that is most at risk of being offensive. Mostly because many expect something more than just an open sandbox, where doing what society says is wrong is mostly the fault of the player, while giving the tools to do just that is just being honest about wanting to give a true sandbox expierience by the dev. Many players expect some kind of achievement system, and THERE is where most of the danger lies to go into the wrong direction, handing out points for acts of vandalism and multiple homicides without context other than "the game rewards me for that behaviour so I'll do it". THIS is the fault of the dev, not the player anymore.