Here are some nudges, in order of nudginess.
Six symmetrically entries, the themers, share a similarity suggested in the title.
] In an earlier version of this puzzle, the prompt asked for “a criminal who sometimes changes letters.”
Changing letters suggests that the key to solving is to re-punctuate the title.
Think of it as “In. Or out.”
If that doesn’t do it, here’s the big shove.
A title even more descriptive of the mechanism would be “OR out. IN in.”
Some entries on first glance seem ripe for this substitution. But a.) they are not symmetrically located, and b.) the substitution does not result in a real word. Ignore them.
You know the drill. Each newly created word can be clued by the clue for another entry in the grid. Those alternate entries — first letters in their order in the grid — spell a six-letter word.
That word, however, is not the answer. To turn it into a type of criminal, you must make a now-familiar substitution