I'm not concerned, these things are so heavily policed and NO government wants to be shamed hosting a major event that they pass off largely fine.
Even in Marseille in 2016, looking at the news you'd think it was mass disorder but really it was isolated to a few streets. The Russian thing was a bit mental, but given everyone said I was going to die, get carjacked, raped etc in South Africa - when actually it was a brilliant world cup and no sign of troubles. The police presence is huge, and I don't expect Russia to be any different.