It is safe to travel to Russia generally, and FIFA World Cup isn’t an exception.
FIFA does everything possible and impossible so that every football fan who is going to watch the games feels safe and secure. On their official website, you can see loads of information about each city so that you get more prepared to how things work in Russia, what to see and to do.
From my point, I know that MeetnGreetMe has English speaking local people in each host city who can help foreign fans navigate around the city, explore it and experience the Russia culture, etc.
I think it was safe to travel to the World Cup in Russia as it is to travel to Paris, New York or London as an example. Bad things can happen anywhere in the world these days - it is very sad but we have to deal with it. And try our best to prevent it from happening.