>>42341836The Silvally were poked, prodded, and all kinds of experimented on before they were locked up and frozen. It makes every bit of sense that they'd be aggressive.
Once Gladion spends some time with it, sure it wouldn't outright attack people, but it would definitely show aggression.
In the game, Gladion was fine and in no danger.
In that scenario, Gladion is desperate and vulnerable while being bullied. Type: Null would have every reason to scare them off.