One possible thesis statement is that Romeo and Creon are both tragic heroes because they make flawed decisions that bring about their destruction; however, Romeo's decision is motivated by love, while Creon's is motivated by regard for the law over the family.
A tragic hero is an otherwise virtuous character who makes a choice that causes his or her ruin. Romeo is a loving man who is already married to Juliet when he and Mercutio stumble upon Tybalt, who is from Juliet's family (rivals of Romeo's family). Romeo at first refuses to fight Tybalt, but in the fight that ensues, Tybalt kills Mercutio. Romeo's mistake is to then be so angered by love for his friend Mercutio that he slays Tybalt. As a result, he is banished from Verona, leading to the misunderstandings that cause his and Juliet's deaths.
Creon in Antigone is also a virtuous leader who makes a wrong choice, as he refuses to allow Antigone to bury her brother, Polynices. He thinks that the law of the state, which brands Polynices as a traitor, is more important that Antigone's familial ties to Polynices. After this decision, Antigone feels like she must disobey Creon, and she is put to death, causing death and tragedy in her wake. Creon is a tragic hero like Romeo but is, unlike Romeo, motivated by the law of the state rather than thinking of the bonds of love and the family.