They were well armed, had an entrenched base of operation, and were bent on armed rebellion using guerrilla tactics as their ultimate goal.
If you trust Sadrist and SCIRI officials, which is all we have to go on right now. They can't even agree on who they killed, so how do they know what they were up to?

And the counterinsurgency kicked their butts. Classic counterinsurgency doctrine. Period. Take it or leave it.
Leaving it, thanks. Counterinsurgency doctrine? These guys weren't beat with counterinsurgency doctrine. They were annihilated by American tanks and air support, after nearly overwhelming the Iraqi forces sent against them. In other words, pretty much a straight-up conventional battle.