The thing is you can feel concern and compassion for the mentally ill, the brainwashed etc. and STILL feel compassion, at least as much compassion, for the victims.
People who say "Well I'd rather feel sympathy for the VICTIMS!" "What about the VICTIMS!!!!???"
Yes of course the victims, obviously. Can't you get your heads round the fact that feeling compassion for a perpetrator - a teenage boy who has been manipulated and now shot dead - doesn't mean you think what he did was fine and you don't give two shits about the victims? You can care about everyone, you know!
IMO a huge problem with ISIS and one of their masterstrokes is that their methods appeal especially to those who have something very wrong in their lives and feel a sense of alienation, may be mentally ill, are suffering and want a sense of power and agency and want to lash out. It gives them justification to lash out and hurt others. It's not to do with Islam, that's just a handy ideology to hang that justification on.
It reminds me of the pilot who crashed the plane into the Alps - very mentally ill, felt the world was against him, wanted to die, wanted to feel important. He happened to have a plane at his disposal so he used that. Others can get sucked into terrorism for similar reasons. I was sorry for him too - he was let down. Yes, and at least as sorry for all the victims.
And there are families of victims in these types of incidents who do show compassion to the perpetrator.