I will take your question, and answer in-line where I can:
A common argument in today's news is that:
1 Someone commits a heinous crime by shooting a bunch of people.
- Only possibly heinous, as it stands it could be a heinous act or an act of compassion, good, necessary evil depending on context and subject
2 Anyone who commits a heinous crime must be insane.
- Again not necessarily: motivation and rationale would determine insanity
3 Sane people cannot apply rational thought to explain what motivates
- Also not necessarily correct: a sane person could apply a rationale which worked to explain the motivations of an insane person at a given moment or for a given action, that however does not mean that the same motivation would stand for all actions.
4 Therefore, one cannot ascribe a cause or catalyst to a heinous
- Again not necessarily correct: if the so-called heinous crime were committed for valid (or even invalid) reasons, there may well be a clear catalyst or cause; whether that clear catalyst or cause is valid or justified would be a separate matter, but it could definitely be clear.
Is there a fallacy in this logic? If so what is it?
Heinous: this word can mean hateful, odious, abominable, totally reprehensible, utterly wicked. None of these necessarily infer that they are random, insensate, irrational or non-cognisant acts.
Insane: this can mean in a state of mind which prevents normal perception, behaviour, or social interaction; seriously mentally ill; possessed of divergent reason and logical processes.
However both these terms are sociologically and societally weighted, and are defined in relation to socially accepted norms, and hence highly subjective and variable by location, era, etc..
The fallacy is innate to the style and is based on the presumptive and assumptive nature of presentation; the language of news and media is primarily driven by sensationalism (some further evidence/reading in regards of media sensationalism:1,2,3,4,5) and propagandism: made overly dramatic and emotive and presented as absolute truism, with little or no presentation of alternative possibilities put in place, unless they aid the sensationalism or propagandism.
As with any communication one should strive to understand the meaning and motivation behind it as well as intended aims, that will lead to a better understanding of what is being communicated as well as how it is being communicated.