Africa has a very long history of not only various factions fighting, but tribal conflicts that go back centuries. Things were only made worse as colonial powers played different tribes against each other. Add to this organized crime, religious hatred, and generally low regard for life, and we have what we have.
Also, "poisoning the well" is a well documented military tactic in European heritage as well. Similarly devastating to the local populations are scorched earth tactics, for example used in WWII.
Indeed, African militias do not have a monopoly on "poisoning the well" or other scorched-earth tactics. For a fantastic Western example, I would humbly point us all in the direction of our own nation's conduct in the Vietnam War, viz., with Agent Orange.
The explicit aim of the Agent Orange campaigns was to render cropland useless, starving rural Vietnamese out of the countryside and driving them into (US-dominated) cities. Entire ecosystems were devastated as a result, and generations of Vietnamese suffered the aftermath.
This was basically the 20th Century equivalent of the old Roman tactic of sewing salt into the enemy's wheat fields.
Also, "poisoning the well" is a well documented military tactic in European heritage as well. Similarly devastating to the local populations are scorched earth tactics, for example used in WWII.