Unfortunately, in such cases as these, the death penalty is pointless. The fanaticals are going to carry out these atrocities without regard for the consequences and the only way to change that is by promoting the right moral values and rooting out the immoderate religious zealots.
So... Since there's "no point" let's not bother? If the consideration of a death penalty deters a single radical from carrying out such an act then it is worthwhile. Trying to talk them out of it is simply naive.