First, even if she is investigating them, undercover agents is a pretty complicated field, while some are exempt of minor crimes it would be extremely hard a jury pardon a murder, and if her investigation is at this point she already has proof to seize them.
To quote one example:
One of the more well-known examples of this is the “Donnie Brasco” case, where undercover FBI agent Joe Pistone spent years infiltrating the Bonnano crime family. He eventually gained their trust to the point that he was going to be made an official “made man” or official member of the mafia. To do this, however, he was required to kill someone, which the FBI couldn’t approve. As such, the operation was shut down (although by that point, they had already obtained enough evidence to convict over 100 mafia members).
Second, her morals, if she thinks an innocenti life for a operation is worth the price she is already a bad and scummy person. Even if we buy the whole "she couldn't arrest them, she stills need proof of their other crimes" she could very well:
1. Tipped Dion off.
2. Arranged an emergency kit for him.
3. Came back later to at least check on him.
4. Involved her superiors.
She had time for ALL of them, the attempt at Dion was premeditated, they talked with her about killing him, set up a plan and she agreed and acted on it.
The author just wanted to subvert the whole "all the other members of the part are comicaly evil" trope but did it in a poor way, i don't know his plans for her after her 'punishment' but i surely dont think he will write any compelling character growth or 'redemption' arc for her