From a tourist’s perspective, Rwanda ranks as one of the most crime-free countries in Africa. Kigali is a very safe city, even at night, provided you follow the same kind of commonsense rules – avoid walking alone in unlit dark alleys, or overt displays of wealth – you would in any large city. Ideally you should leave any expensive jewellery at home, and avoid leaving valuables such as cash, mobile phones and electronic devices lying around openly in your hotel room. Padlocking your luggage will not prevent a determined thief from slashing it open, but it is a strong deterrent to casual light fingers.
It’s a good idea to carry a scan and/or electronic version of all important travel documents, in case they are lost or stolen. You might also want to email all such backups to a webmail address you can access anywhere on the road.