We can not say that all the refugee terrorists, but when you have a large number of unknown people that you do not know anything about them and you do not know their culture and you put them between your citizen then you will have a lot of risks, so they should create a safe areas in there country and put the refugees.
With the sheer mass of refugees entering European countries, it is very difficult to vet them all properly, and some bad eggs are sure to slip through the cracks. Some radicals are adept at forging their documentation and can bypass any safeguards in place. Once within a country, these refugees are often not assimilated into society well, which can further foster feelings of resentment towards the establishment. This creates a more fertile ground for acts of terrorism to be committed within the borders of these countries.
The mistakes made in Germany and EU concerning refugees created a more dangerous situation for their citizens and the rest of the world. Unfortunately, without strict admission policies in place, terrorists can easily come into their country and pose a danger to their citizens and the rest of the world.
No, they probably did not create a more dangerous situation with terrorists. Merkel admits mistakes were made in Germany and EU concerning refugees. However, this is a situation involving migrants, and not necessarily terrorists. There is not proof that this created a more dangerous situation with terrorists, so we cannot assume that it did.