That’s my question too. For Harry to end up under the Doom of Damocles rather than executed his killing of Justin would have to be seen as justified and that he could be redeemed from his use of black magic.
So, either Justin was already suspected or someone saw some incredibly damning evidence when they caught Harry. Either way if Justin was the guy given the responsibility for destroying Bob you’d think after Harry’s trial people would be asking questions about his career as a warden.
I think the Wardens didn't know Justin had turned. A Warden-gone-Warlock is a SERIOUS threat... knows too many secrets, too much Council & Warden security processes, etc. They would have HAD to put him near the very top of their "most wanted" list.
And then for him to be as active as he was? Monitoring young Dresden (and Mallory) for magical gifts, adopting them from their agencies, training TWO apprentices while they attend school, etc... I see this as a huge security risk for a known & hunted Warlock!
As an ex-Warden & WC wizard in good standing, nobody's looking for him, monitoring his activities. He could EASILY have done all that without anyone particularly noticing (or caring).
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Then Harry kills him.
If Justin was a known Warlock, the "Self Defense" plea is entirely credible, even likely. A known Warden-gone-Warlock would probably already have been under a death-penalty order, with a 'kill on sight, too dangerous to attempt to capture" clause.
I don't see Harry's trial being such a close call, had DuMorne been a known Warlock; more a matter of "well, YEAH... of COURSE the kid had to kill him! And lucky to survive! Soulgaze to verify, but SelfDefense is the only account that makes any sense."
OTOH, there must have been SOME hard evidence that Justin HAD turned. If all to the known facts pointed to Justin being 100% clean, then Harry's allegation of "Self Defense" wouldn't have been believed at all, because nobody would have believed that Justin Lilywhite had tried to kill Harry!
So, not a known Warlock, but still credible as attacker, demon-summoner, mind-controller...?
My guess is that
enough of Justin's lab survived that the Wardens saw plenty of evidence that he had gone Warlock... and had been for some time. Plus the signs of the recent psychic attack on Harry -- Justin's attempted Enthrallment -- would have been clear, further proving that Justin had both ATTACKED him, and had used Black Magic in doing it.