NEW THEORY
I'm so thrilled about this new S5 theory that has been going around about this clip:
The inicial theory was Will feeling Vecna taking over his body again (getting possesed), so he screams at everyone to get away from him to not hurt anybody. This could make sense but i highly doubt Will's gonna get possesed again like in S2, besides, in that season his voice when he was possesed and screaming didn't sound like him at all, it sounded like a monster, distorted, and in this clip is 100% Will´s voice screaming.
Now, there's another theory which makes more sense (at least to me).
As we all know, this sound was leaked before the teaser came out, and it says something like:
"Steve/Mike! (someone's name), Do you hear me? You need to run! Run!"
When Chrissy faces Vecna, Eddia also asks "Do you hear me?" and when Max is under Vecna's curse she gets away by running and escaping. Someone might be under Vecna's curse levitating right in front of Will, presumably someone he rally cares about like Mike, Joyce or a third less probably option Jonathan. He's that terrified cause he knows what Vecna might do to that person if they don't get away in time thanks to his conection.
