Why Do We Judge People on Their Intentions?

We often reward and punish people for their actions, whereas we hardly ever do this because of their intentions. If a would-be murderer doesn't even get to the stage of attempting the murder, then he is guilty of no crime according to Abilard. The road to hell is famously paved with good intentions. But if someone does something truly appalling while intending to carry out God's will? In that sort of case, wouldn't we condemn the appalling action despite the person being doing their best to act well?