Still too simple, yeah a powerful wizard having an emotional violent reaction to his beloved being senselessly killed before his eyes can do a lot of damage.... You might even call him a destroyer, but is he? He can destroy, true, but I think your example is a poor one because of the context. To be a true destroyer in my opinion, it takes planning.. It is more like damage verses collateral damage, a destroyer will plan to flatten that hospital, verses someone trying to take out a missile site and a hospital is accidentally destroyed on the process.. Either way the hospital is screwed, but in the first case it was done deliberately by a destroyer, and the other was done accidentally by some capable of destroying, but isn't a destroyer.
I think a Destroyer has to be a
Starborn.
With the right impetus, any wizard in the grip of rage & despair might Open the Outer Gates.
A Starborn... who knows?
Maybe they can open the Outer Gates such that nobody can close them again, or summon all the Walkers at once, or summon ranks above even the Wakers, or...?
We don't know much about the Starborn; only that they wield power over the Outsiders that other wizards cannot.
Limits to Starborn power? No clue...
We saw in Changes, how Harry cold-bloodedly used Molly -- used her love for him -- to guilt her into a violently self-destructive spiral; if Harry's plan (for her to assist his suicide) had succeeded, Molly would have died, or gone full-warlock. After the fact (in Ghost Story and in Cold Days) he realized how wrong -- how evil -- that was. But he knew before that, when he asked her; he just wouldn't/couldn't allow himself to look the facts in the face, because he "needed" her to do that.
We saw him ready willing & able to do murder after Rudy shot Murphy; only the KotC's (all of them together!) saved him (I still think this is evidence of Denarian influence on Harry).
So yeah, I think if Harry "snaps," and brings all his power to bear, and if the Starborn thing can leverage the way I suspect... Harry would be capable of making the Harry-Prime universe one that simply has to get Pruned (by Uriel etc).
Destroyer enuf 4 me !