A complex sentence contains one independent clause and at least one dependent clause. The difference between an independent clause and a dependent one is that the former can stand alone as a sentence as it represents a complete thought. The ungrammaticality of sentence A proves this point. As you can see, without the independent clause (B), the dependent clause (A) does not represent a complete idea, it depends on the main clause to complete its meaning (C).
A) *Because it is cruel
B) The death penalty should be abolished
C) Because it is cruel, the death penalty should be abolished
