Let's not beat around the bush here: Henry feared/hated his father. Now, yes, Butch and Henry did have their moments, but Butch was mostly harsh on his son (which can be seen in the movie as well). Pennywise probably rationalised Henry killing his father in a bid to say "you scratch my back, and I'll scratch your's, buddy".
No Butch = no rules/beatings for Henry; an added bonus which he can enjoy after finishing off the Losers for good.
These quotes seem to suggest this:
Henry placed the business-end of the switchknife against his father’s scrawny neck.[...] Henry kept the knife like that
for almost five minutes, his eyes distant and thoughtful, the ball of his left thumb caressing the
silver button set into the switchblade’s neck. The voice from the moon [It] spoke to him—it
whispered like the spring wind which is warm with a cold blade buried somewhere in its
middle, it buzzed like a paper nest full of roused hornets, it huckstered like a hoarse politician.
Everything the voice said seemed pretty much okey-dokey to Henry and so he pushed the
silver button.
...
He [Henry] would kill them all, and the voices—those inside and the one which spoke to him from
the moon—would leave him alone. He would kill them and then go back to the house and sit
on the back porch with his father’s souvenir Jap sword across his lap. He would drink one of
his father’s Rheingolds. He would listen to the radio, too, but no baseball.
It, In the Watches of the Night, 8
Ultimately, I would see it more so as an incentive for Henry doing It's dirty work for them.
