Hopefully this article will clarify in a few minds why the Spanish Catalan police shot dead the terrorist when he screamed "Allah Akhbar!" while wearing what looked like a belt with explosives. 
When dealing with terrorists who are ready to immolate themselves in order to kill others, there is little time for the security forces to react; and they have no time to verify whether a suicide explosives belt worn by the purported terrorist is a fake or not, or to read him his rights under the Constitution. This is a very serious business, a matter of life and death. Taking an extra split second to make a decision could be the difference between the death of the terrorist(s) and the death of many agents and bystanders.