Frank recognises his mistake. He let Nicholas Angel live too long.
You nip blight in the bud: hoodies, jugglers, by-the-book Met sergeants. You don't let it get established. You protect your town.
Protect your son, even from himself. Danny liked Angel, and Frank wanted him to have a friend. He didn't see the corruption, even when Danny betrayed him.
If he'd known what it meant when Danny wouldn't step away from Angel . . . if he'd foreseen Danny visiting to announce he's a shirtlifter and proud of it . . . well.
What blight takes, you have to cull.