People are being mugged by citizens of Brazil, and if the natural state of the country is that bad it's crazy to think how bad the international terrorism concerns must be. They should just cancel the Olympics at this point as they're certainly not looking like they'll bring anything good to the table.
Yes, there is a high security concern for the Rio Olympics just as there is for every Olympics. This does not mean that the Olympics will not be safe. Even with the security concern, the Olympics in Rio should be as safe as any other Olympics even though there was an explosion at the site of the Olympic Opening Ceremony.
There have been issues with security and crime well before the olympic games started. This explosion is confirmation that those fears were well founded. There have been other smaller incidents, and this latest one is further proof that there is reason to be afraid and that security needs to be increased.
In my opinion, the Rio Olympics have huge security concerns, mainly due to poor preparation by organisers. The private security firm due to handle the event were fired weeks before the opening ceremony, being replaced by soldiers and ex-police, leaving the event open to outside threats. Because of the tense political situation in Brazil currently, there are likely to be many protesters who want to make their voice heard at the event.