Tanzania is generally safe to visit, though there is the risk of violence from gangs, crime, and terrorists, as is the case in many places in Africa. Please check your country’s travel advisories for more information (you can view Canada’s Travel Advisories for Tanzania here).
Once you get onto Mt. Kilimanjaro, the threat of this level of violence drops dramatically. However, there are risks inherent to climbing and trekking, and people do get injured on this mountain. But as mountain climbing goes, Kilimanjaro is challenging but not particularly difficult from a technical mountaineering perspective.
There is little threat from wildlife on Kilimanjaro. Snakes exist on the mountain but are rarely seen. Lions, water buffalo, hippos, and other sometimes-dangerous animals are rare-to-non-existent at the base of the mountain and non-existent as you begin to climb. Mosquitos (and the Malaria they can carry) are the biggest risk at lower elevations.