I'd be more concerned about the 14 year old thinking it is scary than the 10 year old for thinking it is cool. At 10 they don't fully grasp the devastation of violence and war, or reality of death, in a more sophisticated way like teens and adults. So it doesn't mean the same thing to him as it does to older kids and adults. To him it could have been like a game or perhaps he understands that it is merely a representation of something rather than people really getting killed in front of his eyes. It isn't going to seem as serious to him as it will to people who are older.
If guns are something he has been taught to use and enjoy, it could send a mixed message to him that on one hand they are good, and on the other they are bad and he shouldn't have liked it. He might have found it appealing that a younger boy was taught and encouraged to help fight, and felt a sense of empowerment and the illusion that he could contribute in a meaningful way. However, an issue is made of him enjoying something you put in front of him.
For your other son, at the age of 14, I think he should have a more mature understanding of death and war. It would be reasonable for it to effect him, but for it to frighten him is a bit concerning, especially if he had his eyes covered and was not really seeing much. Perhaps your children could use some more exposure through movies, books, and conversation so that they can cope with the harsh realities of life. Maybe the fear was something he adopted because of how protective you were over what he saw, "if mom doesn't want me to see it, then it must be really bad!" By not wanting the children to see it, you are sending the message that they can't handle it, and some kids will be exactly the way they think you believe they are. If you think they can do something, they believe they can. If you think they can't, they believe they can't. If you think something will traumatize them, they believe it will.
side note: I personally do not approve of children using guns, and believe that our society has too much unchecked gun violence. That's all I'm going to say about that. Other are entitled to their opinion, and I respect that. I won't debate it.