What was Big Willy Shakes going for?
When Antonio says "lie low," he's actually talking about dying, not chillin'. To lie low was to be killed, or to be found dead in those wayback days. Low indeed.
In fact, Antonio means this to be a threat to the other men. "You better watch it," he's saying. Instead of telling the men to take it easy, he's actually saying that they shouldn't mess with Leonato, otherwise they could wind up dead. Leonato's got a lot of anger and nowhere to put it. If the men even look at him in the wrong way, it could end up deadly for them.
Pride is a big deal in Much Ado About Nothing—not because any of the characters suffer from excesses of it, but mostly because characters are susceptible when their pride is wounded. Pride is damaged and preyed upon more often than it's inflated in this play.
In this scene, Leonato is bummed because his pride in his daughter and his family is wounded. She's been accused of the worst possible thing for a 16th century gal. He's super depressed about it. He's so down in the dumps he'll pick a fight with just about anybody.
Antonio warns the other men about this to tell them to get out of Leonato's way. Haven't they done enough damage accusing poor Hero of despicable things?