The first and most obvious way that electricity can be harmful to human life is through accidental electrocution. Electrical currents sent suddenly through the human body, which itself contains electrical signals, at a high-enough voltage will kill instantly. The sudden jolt of electricity, for example, through accidental contact with a live electrical wire like a power line leading into a building causes massive trauma to the cardiovascular and neurological systems. The heart then shuts down quickly.
Another, more wide-ranging and long-term way in which electricity can be harmful to human life is through its mere production. Coal-fired electrical plants generate more than 40 percent of the world's electricity while also contributing to the production of greenhouse gases believed by many to be the leading cause of global climate change. To the extent that higher carbon dioxide levels in the atmosphere is harmful to human life, then electricity generated by coal-fired plants is harmful to human life.
A third way that electricity can allegedly harm human life is through the radiation emitted from overhead power lines. While the existence of such power lines, which emit electromagnetic radiation, is not considered harmful to the population at large, it is advisable to not live in very close proximity to them. Power lines are not particularly dangerous because of the low levels of ionizing radiation they emit in unconcentrated form; spending a disproportionate amount of time under them, however, could result in a gradual and unhealthy level of exposure. As noted with example #1 above, electrical currents that penetrate the human body from outside interfere with the body's own electrical system. Sustained exposure to even low levels of electromagnetic radiation, then, can cause life-threatening illnesses.
Those are three ways that electricity can be harmful to human life. A fourth, just because, as with accidental electrocution, it is quite obvious, involves lightening. Lightening, of course, is electricity discharged at a massively high level and in a very concentrated form that can kill instantly if it directly strikes an individual, and can kill many people if it strikes the metal shed in which they are shielding themselves from rain.