If you still decide to visit a favela, before you go there, download this app on your phone (I assume that you have a smart phone). You can download the app now from your home location.
OTT (Onde Tem Tiroteio)
No matter where you live in most other countries, the app will function and send you a notice each and every time a shooting occurs if the shooting is reported to the police in Rio-
The app will also tell you the time and location (neighborhood) of the shooting.
You will notice that most of the shootings occur in the favelas.
After you download the app and wait a day or so, your phone fills with countless alerts. After you see the actual alerts, you might want to reevaluate if you still want to visit a favela at this time in Rio.
I suspect the people who told you that seeing a favela is a must, are involved with selling favela tours.
Yes, even if you are located in Australia, you will receive the alerts.
Not a pretty picture.