When the roof hatch is breached, the perp is adding the ricin to the A/C unit while wearing a full-body haz-mat suit, including hood, full-face respirator, and SCBA air tank. He flees the SWAT team, and jumps to an adjoining roof wearing only the basic yellow suit. With only seconds having elapsed, he had no time to discard the extra equipment.
Cutting wires in a panel labeled "AC CONTROL PANEL" would not take out as much power as indicated, especially not the lighting in a computer room. Further, if that much power was impacted than the arc flash, fireball, would have been very impressive and put the officers in the hospital. Opening the breakers would have been faster and more accurate than cutting unknown wires.