But all the while, he lived in the guesthouse where authorities believe he turned from con man to killer. (source)
This seems to be a case of article elision. Why can articles be dropped before "con man" and "killer"?
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The construction verb from X to Y uses nouns in their generic meaning for X and Y, even when either X or Y, alone, would require an article. Con man and killer wouldn't ordinarily be used in a generic sense alone, except with carefully selected verbs.
You could say he was a con man and turned into a killer. In each case, he is an example of a con man or a killer, so an article is used. You couldn't say "he was con man" or "he became killer", using the terms in a generic sense.
But it does work with certain verbs, "turned" being one: "he turned con man" or "he turned killer".
To the phrase in the question, the construction he turned from con man to killer lets you use each term as a generic class, so no articles are needed.
Some other examples:
the house went from dump to showcase
the car went from clunker to hot rod
the meth addict went from movie star to bum
the plane lost its wings and went from glider to rock
he grew from boy to man
On pp.409–10 in The Cambridge Grammar of the English Language (CGEL) by Huddleston, Pullum, et al. (2002), in Ch. 5, §8.5 titled Restricted non-referential interpretations of bare NPs, subsection (b) Fixed expressions or frames, there are listed some categories of special constructions, and the examples there look similar to yours:
arm in arm, back to back, day after day, mouthful by mouthful, [repeated nouns] side by side, mile upon mile
from father to son, from beginning to end, between husband and [matched nouns] wife, mother and child
[...] The examples [above] are illustrative of a number of expressions involving repetition of the same noun or contrasting nouns [...]. Similarly, in coordinate structures, bare NPs can optionally be used in repetition: We searched endlessly for a spring or a cave to spend the night, but neither spring nor cave could be found.
CGEL lists and categorizes expressions such as in hospital, by bicycle, have lunch, at dawn, but I didn't include these and the accompanying comments as they don't quite look like the example in the question.
Anyway, I asked professor Pullum (and one of two coauthors of the relevant chapter, Payne, as it turned out: they seem to have been in the same place at the right time) and he told me your expression had to be exemplifying one such special construction. He commented that there are many exceptions to the default requirement that an NP with a singular countable noun as head have a determiner, and then listed a few such examples (e.g., with hand on heart, tongue in cheek).
It may be an elision. But, depending on how you look at it, the nouns could be acting as adjectives, modifying him.
Consider the ambiguous:
That is a safe house.
It could be interpreted as there being a noun phrase safe house, or as there being a noun house that is modified by the adjective safe.
Semantically, you can have a killer recipe, so (theoretically, although it seems ungrammatical with the words in this order), you could have a killer man.
In your sentence, if there is elision, I believe the elision would actually be being a rather than just a:
. . . he turned from being a con man to being a killer.
I don't think it is article elision but article omission. It is not an elision in the linguistic sense.
The answer is simple: there is no article because it's making a general statement.
Adding an article would make it refer to a specific person, idea, object or concept:
... from the con man to the killer.
If there was an article that was omitted or if it were to include an article, it would be the indefinite article a:
... from a con man to a killer.
Because it would have the same meaning as:
... from con man to killer.
The latter is preferred, because "from a A to a B" just seems tautological, when it can just be "from A to B". There are certain exceptions though, that being a matter of style--the article may be useful for emphasis.