Air Cannon.
In that instance, he also reached out towards Izuku and Katsuki before they could react. They are
fully aware of how Decay works and how All For One works. Tomura catching them off-guard in his current state, or in any state considering how powerful Decay is, is the worst case scenario.
Why on earth would that scare him? He used it to disable communications, not as an attack, and the destruction that stemmed from it is ridiculously less than even AFO did at Kamino. And even then, why would a display of power like that even REMOTELY scare Bakugo, who does bigger attacks on that since the sports festival? Bakugo has never seen AFO’s Air Cannon, so he has no frame of reference to even be scared of it, and he has never seen decay. How is Bakugo hallucinating himself dying to decay when he has never seen decay ever before, not even at the USJ or when he was trapped by the League.
You are taking those pictures not only out of context, but not even in the right order.
Shigaraki does his air cannon+radio waves to disable communications, NOT to attack anyone, and thus gets propelled in front of Deku and Bakugo. Deku and Bakugo are IMMEDIATELY sent into fear by his presence, before he even talks to them or points his hand at them. This is shown with Deku already beginning to crumble before it even gets to the next panel. Then they hallucinate their deaths, then he talks to them.
Also, worse case scenario is enough to make Bakugo, one of the most headstrong, willful, battle hungry and eager to prove himself characters out there, who CAME TO THE BATTLEFIELD SPECIFICALLY TO FIND AND ATTACK SHIGARAKI, have a hallucination of his death from sheer fear at merely seeing him???? That makes absolutely no sense whatsoever and requires a hefty amount of evidence to prove. Bakugo has 0 reason to be afraid of Shigaraki here, yet he is. Deku IN THE PANEL YOU SENT is attributing this sensation they’re feeling to be the fear of death, something you don’t get just because someone you had the intention of fighting appears next to you.
Shigaraki’s power isn’t even something Bakugo hasn’t seen before, he literally fought All Might whose first punch was more destructive than the Air Cannon Shigaraki did. Attempting to claim that a basically harmless attack aimed at no one was enough to make BAKUGO KATSUKI afraid to the point of seeing his own death in front of him is ludicrous. Where is ANY evidence to suggest that is what caused the fear.