For all of Inigo's panic, there's a reason why he's clinging to Roland specifically. It's since he knows Roland will make it all better. That's exactly why Inigo sees him as a father figure, after all - Roland always knows what to do, can keep calm in any scary situation. He will take care of this bug without hesitation or fear-- or even with a teenage boy clinging to him, probably.

So even though Inigo feels his heart racing as he clings to Roland, shutting his eyes tightly as if he doesn't even want to see the bug anymore before it's gone, his heartrate slows down again after he hears the gunshot. He opens his eyes, seeing the expected view of nothing but the bullet mark on the wall. No more terrifying bug.

Phew.

That's an actual out loud 'phew', in fact, since Inigo sighs in relief. Slowly he releases his death grip on the other guy, calming down. It's all okay now. The beast has been slain.

..

Until there's suddenly a buzzing sound, and the cockroach flies right past Inigo's head.

The boy lets out a loud shriek and then runs straight ahead in an attempt to get away from the ungodly creature.

Unfortunately 'straight ahead' happens to mean 'right into a wall' in this case. Inigo hits the stone face-first, and the force is enough to knock him right down, lying on his back on the ground.