tautology

      There are so many ways to be wrong, that to define them formally, as in a class on logic, they have each been given names.   
       A tautology is the formal name for a logical error--the error of using a thing to explain a thing. "He killed them because he's a serial killer" is a tautology. It is not an explanation, although it sounds something like one. Serial killers kill people, so that part is true. But illogical. He didn't kill them because he's a serial killer. The because renders the sentence illogical. "He killed them because he's a serial killer" makes no more sense than "it's raining because it's wet out". It may be raining out, and wet, but it isn't raining because it's wet. Wet is not the cause of the rain. Serial killing is not the cause of the murder.

a common tautology

     A "tautology" is a classic error of logic--one on the list students were once taught to recognize and avoid. Someone using a tautology defines the thing or idea in question by defining the thing or idea in question--"it is what it is" is a tautology. A definition can make a sound  basis for an argument, but it is not a substitute for an explanation. A common tautology, or set of tautologies, that you can hear on TV crime shows sounds like this--
Question--"why did the suspect do these crazy things?"
Answer--"because he's crazy."--or "because he's a psychopath."
The "why" part of the question has not been answered.
Illogical.

The "argument by definition", in the case of the TV psychopath, would not be particularly useful, but would run like this--
A psychopath is defined as someone who commits violent crime for no discernible motive. He has no motive of profit, and no motive of protecting his reputation, two of the most common criminal motives. This criminal commits violent crime without any discernible motive, therefore this criminal is a psychopath.
 Logical, but not particularly useful.