That's the thing, there can't just be a million different definitions of the word "terrorism," as it would invalidate the term. It cannot be just "murder", as it would take away the horror associated with killing for political/social purposes.
But wouldn't that also serve to invalidate the term "terrorist"? You say that "it would take away the horror," but what does that really mean? Does this mean that the word "terrorist" is used more to evoke an emotional reaction, rather than any attempt at accuracy or clarity? And if the word is used strictly for the sake of emotion, how is not already invalidated on that basis alone?
You are, of course, welcome to think of anything as terrorism, but there must be a dividing point based on intent. If someone kills in the Mafia, they are usually driven by money or power. I would say that could be seen as a political objective, so maybe the "terrorist" classification is warranted. But, that is very different from someone killing someone while robbing a bank or house, as the intent is not nearly as deplorable.
In short, I feel it necessary to punish those that use violence to further social (religion is included in this) or political objectives more severely.
I believe that they can and will be punished more severely based on the severity of their crimes. Intent and motive might also be taken into consideration. But that doesn't mean the language has to be clouded or used more for emotional sake rather than for accuracy. Your OP pointed up one of the many traps and pitfalls in doing so.
As for what may be more "deplorable," that's a value judgment based on perception. If someone kills my friend or relative robbing a bank or house, that might be more deplorable to
me than if someone killed people who are total strangers to me. Some might believe that serial killers or child molesters might be the
most deplorable, even more deplorable than terrorists. Who's to say what's worse?
It's also interesting to differentiate between "violence with a political aim" versus "violence without a political aim." That might mean, if one is a "rebel without a cause," one can be as cool as James Dean. But a "rebel
with a cause" would be a terrorist.