Leftists are experts at ignoring the law of Occam's Razor. If someone came up to me and stuck a knife in my chest, I wouldn't blame the knife manufacturer. Leftists would. For liberals there is always the ulterior motive, the underlying reason, the root cause, etc. Leftists can never bring themselves to blame the direct perpetrator for a crime or crimes. In this case, and hundreds of others, it is Muslim fanatics. The left should admit it and deal with it. But they can't...because they're leftists.
I have the same reason for wanting to see Andrea Yates executed for murdering her five children. I could care less about her being mentally ill or mentally deranged. She crossed the line when she killed the first one but then went on to kill the four others.
But if a mentally ill person was guilty of constant shoplifting I would want society to cut him or her some slack and try to help them. In that far more mild crime I am willing to consider motive in sentencing. But not for murders