It was a dark night + , or

It was a dark night, a prisoner of war who had intended to escape killed the guard and changed into his uniform.

Why not ‘It was a dark night. A prisoner...’?

Solution 1:

The sentence has two independent clauses. Using a comma between them, without a conjunction, is an example of a comma splice. Some comma splices are acceptable—either those that have commas separating very short clauses, or those that are used for intentional stylistic effect. But none of that seems to apply here.

There are several ways it could be repunctuated or rephrased. The more common (including your suggestion) follow:

It was a dark night, and a prisoner  . . .
It was a dark night; a prisoner  . . .
It was a dark night. A prisoner . . .

It could even be rephrased into a sentence with a single clause:

It was on a dark night that a prisoner . . .