[inspired by the Darkhallow Cowl vs Ethniu thread]
In GP, Cowl gave Nemesis to Lea, weakening Winter at a critical time.
In DB, Cowl never intended to successfully complete a Darkhallow - DB turned out pretty much exactly as Cowl intended. That's why he was able to escape the collapsing Darkhallow; he'd planned it out ahead of time.
The result? The White Council was very severely weakened, but the Red Court also overextended itself, expecting Edinburgh to fall ... setting things up for Changes.
WN also went mostly as Cowl intended -- that plan seems precisely designed to get Harry to stop it. Harry's chivalrous streak is pretty well known, as is his working outside the Council's traditional scope and with mortal authorities; if they really intended a plan based on attacking non-Council-member magical women to succeed, Chicago would be the absolute last place on the planet to try it.
But at the very end, Harry did something Cowl didn't plan for - working with Lara to get out. I think he probably expected Harry to escape through the NN. Lara wasn't meant to survive - the White Court leadership was intended to be totally destroyed, ending it as a relevant political force. (Instead, it's actually probably stronger in practice since now united under Lara).
Cowl's long-term goal is to weaken all the existing supernatural power structures, break the Accords, call in the Outsiders - and then betray them. Per WOJ, the highest levels of magic can rewrite reality. With the Outsiders weakening the fabric of reality, Cowl will do that: end death, give himself godlike powers, and then kick the Outsiders back Outside.
IMO Cowl has very intentionally not killed Harry. To Cowl, Harry is kind of an "ace in the hole" to ensure that the "betray the Outsiders" part of the plan doesn't go wrong - without ever being consciously helping Cowl, he'll fight to get the Outsiders out of reality.
