Is every terror attack for political reasons justified then?
YES
Who defines what terror is? What is politics? If the policies that govern a society aren’t working for a group, and they are given no other recourse, what are they to do? What justifies anything? Can you justify dropping a nuclear bomb on an unaware city to “end the war?” You can, but it erodes your moral authority to other groups.
“right to kill everybody?”
Not everybody. Violence should only be used if it is necessary to achieve goals. There is a reason Nelson Mandela refused to renounce violent struggle.