At the point when Romeo murders Tybalt, we see a common character trait of Romeo: impulsiveness. This means he acts quickly out of emotion. At all other times we have seen him act impulsive when he went to the party and within moments of seeing Juliet went and kissed her.
Another couple of traits we see of Romeo's in this moment are his defensiveness and loyalty. It is a good friend who will seek to give justice when a friend has been wronged by an enemy. But the problem was that killing is always wrong. To commit a crime on behalf of another shows great loyalty.
You could also call Romeo superstitious. He comments after the murder that he is fortune's fool. He is referring to fate or destiny and how chance has just defeated him. He had his whole world being set up right around him, but then it all came crashing down.