The origins of the war began when colonists bean protesting aganst the Colonial Administration Authortys beurocratic oversght over the colonies. After ma failed negotiations, some protesters grouped together and started using violince, beginning the Insurrection. The violence continued to escalate, and the UNSC soon realized if they do not intervene soon, then the Rebels would take over the Outer Colonies, and lead to a massive civil war between the Outer and Inner colonies.
The UNSC began to send forces to fight the rebels, and soon death tolls, including civilians, steadily rised. With civilins being killed in crossfire, the Rebels had rallying point, and gained more suport, so the UNSC had to change tactics.
The UNSC initialised the ORION (Spartan I) Project, overseen by Halsey, to use small special forces of Super Soldiers in order to limit UNSC and civilian casualties, however it was deemed a failure.
After a few years of escalating war, the tried again wit the Spartan II Project. Halsey wantedo eliminate the problems with the ORION project, one of which was the agof the candidates. The young candidates werselected, kidnapped, and replced with clone (who later died). We all know the rest of the story.
As far as Halsey being a monster is concerned, she was far frm one. She held great guilt and sorrow of destorying the candidates lives, forcing them into a life of war. She felt it was her fault for evey Spartan who died. Se also wanted to protect them to fogive that, developing the MJOLNIR armor for their use.