Only al-Sistani´s death could now be a greater blow to United States
plans in Iraq than Muqtada al-Sadr´s. Bluntly speaking, if they kill him,
they lose the support of pretty much all of the religious Shiia´s, even
those that don´t especially like him and his tactics.

If Americans kill him, they lose Iraq. I rather doubt that the American
forces on the ground understand this; he would be a huge security
threat as a prisoner, but as a martyr, coming from a family of martyrs...

They should have let the Shiia´s themselves to defuse him.