Romeo’s decision to involve himself in the continued fighting between the two houses leads to his ultimate undoing. Avenging his friend’s death by murdering Tybalt causes a deadly strain to exist between him and Juliet. The murdering of his bride’s cousin sets a downward gyre, an emotional mess spiraling out of control.

The event in Act III scene i that causes the most problems for Romeo and Juliet is when Romeo states, "Gentlemen, for shame! forbear this outrage!--" and steps between Mercutio and Tybalt to break up the fight. It is at this point that Romeo blocks Mercutio's defense and unintentionally gives Tybalt a chance to stab and ultimately kill Mercutio.

If Romeo were to have just stood aside and attempted to plead with Tybalt and Mercutio to stop their fighting, Mercutio would have been able to end the battle himself and Romeo would not have been blamed for the outcome and wouldn't have had to kill Tybalt in revenge of Mercutio's death.

Then he would not have been banished and the only problem Romeo and Juliet would have is breaking the news of the marriage to their parents.

Well i think that clearly the death of Tybalt causes the most problems for Romeo and Juliet. Clearly once Romeo kills a member of Juliet's family, her family will never give consult to the marriage. And again, when Tybalt dies, Romeo knows that he will be punished, and the usual punishment is death. Tybalt's death basically ensures that Romeo and Juliet cannot get married.