Prior to the Iraq War, the war in Libya, and any intervention we may or may not undertake in Syria, some hawks insistently argue(d) that there is a humanitarian imperative to step into the breach.
Their arguments can be powerful.
Innocent people are dying at the hands of a tyrant. We have the most powerful military on earth. If we do nothing, the slaughter will continue. And don't most of us agree that some military interventions, like the one that stopped the Holocaust, would've been justified on purely humanitarian grounds, even if stopping the death camps wasn't the rationale for WWII at the time?