What confidence might Mercutio have to step in place of Romeo?
Mercutio seems to be looking for a fight from the very first moment of Act 3, Scene 1, and Benvolio - even as the scene opens - is trying to calm him down:
I pray thee, good Mercutio, let's retire.
The day is hot, the Capulets abroad.
And if we meet, we shall not scape a brawl,
For now, these hot days, is the mad blood stirring.
Thou art like one of these fellows that, when he enters the confines of a tavern, claps me his sword upon the table and says ‘God send me no need of thee!’
Mercutio throughout the whole play is just full of self-confidence, and, even though Tybalt doesn't show any signs of looking for a fight with any Montague (he's specifically - even courteously - asking for Romeo) Mercutio goads him:
Here's my fiddlestick; here's that shall make you dance.
Men's eyes were made to look, and let them gaze.
I will not budge for no man's pleasure, I.
Mercutio is then disgusted by Romeo's (as he thinks, cowardly) refusal to fight Tybalt, and steps in. It seems to me that Mercutio has been wanting to fight a Capulet from the very start:
O calm, dishonourable, vile submission!
Alla stoccata carries it away.
Draws his sword.
Tybalt, you ratcatcher, will you walk?
And, after getting himself into it - at no-one's urging but his own - Mercutio then gets a lot more than he bargained for.