First of all, you'd be hard pressed to find a true "loner criminal". When you read that a "loner" committed some terrible act generally that person is falsely labeled. What is stated as "loner" is usually a misrepresentation of an "outcast".
The difference is that loners enjoy and choose to be alone. They do not care what the world thinks of them and will find solace in solitude. For them, being alone is the most natural thing in the world and, in their solitude, they will not develop the ill feelings and desperation that will lead them to seek acts of violence.
The "outcast" however is someone that is forced to be alone. This can happen for any number of reasons: obnoxiousness, heavy drinking, mental illness - or in the case of Theodore Kaczynski the Unabomber - gifted intelligence. Because of their natural inclination to extroversion, their mental health is dependent on the company that they cannot attain and their solitude harbors frustration and hatred, which climaxes in violent acts.
Now because these "outcasts" are seen to be alone during or before they commit these crimes, they are falsely dubbed by the media and criminal profilers as being loners. Newspapers would even call such a person a "loner" and "lonely" at the same time, which is a blatant contradiction.
People aren't destined to be criminals. But, things like child abuse, alienation and poor choices tend to shape an individual into a less-than-ideal citizen.