He was sad and lovesick about another girl when he snuck into the Capulet's party and met Juliet the first time. This was his first bad decision and shows he is too quick and emotional. He was also speeding when it comes to supposedly falling in love with Juliet when it was just lust.
Romeo killed Tybalt in an act of revenge which also shows he acted too quick. He killed Tybalt because he becomes angry and loses control and kills Tybalt out of Revenge at the same time Romeo is probably lashing out at Tybalt due to the only guilt for having contribute to Mercutio's death. If Romeo had not killed Tybalt, he would not have been exiled leaving Juliet alone.
Some will argue that friar laurence is the blame but that's not the case If Romeo would have watched his actions and weighed them thoroughly then there could be a probability that they wouldn't have killed themselves. When Romeo found out Juliet was dead, he hurried back to Verona right away. He was risking his life just by doing that. People might agree that it isn't Romeos fault but if he hadn't just thought about his actions thoroughly they would have made the best decision for both of them Finally, if Romeo had not killed himself so soon after seeing Juliet lying dead, everything could have been