I think it is very improbable that any western European country would enact capital punishment for this kind of crime. They have bought into moral relativism so much that they no longer can envision a death sentence as being just in any circumstance. And, of course, they don't buy the straight deterrence argument--they didn't buy it for Pershing missiles, so why should they buy it as a way to prevent murder?
Capital punishment won't work against islamofascist killers anyway. They would just consider it martyrdom and a ticket to 72 virgins.
It would only work if they knew they would be buried in a pigskin in the kneeling position, with their a$$ toward mecca.
Then it would work.
If Western Europe can't envision death sentences, they'll have to build internment camps or come up with a pill that wipes out hate and violent impulses. Or lobotomies.