I would say that Xanxus holds onto his slights much too tightly. When he finds out he cannot inherit the family he plots to kill his father. While he does not succeed, his anger continues to fester and brew inside of him which results in a second attempt on his father's life. This second attempt is the ring battles in which he also tries to kill the last candidate that can prevent him from taking over the family purely for the sake of his injured pride. In fact, this attempt on his father's life is not only that! He tries to frame Sawada and then exact revenge for the damage on the Ninth's life at the hands of the tenth boss candidate. He sets it up so that the Ninth should die at the hands of Sawada, in which Xanxus ultimately wins, avenges his father, and gets what he wants out of life - the Vongola family. That's not all! Sawada Tsunayoshi becomes a very sore subject with Xanxus and the name is not even supposed to be uttered in his presence. Dino makes it a point to invoke Tsuna's name during the representative battles to prove how out matched his pupil is in comparison to Xanxus. The mentioning of Tsuna's name sends Xanxus into a rage and he destroys part of the hotel that he's staying in. His defeat at Sawada's hands is still a sore subject ten years in the future, in which he finally loses his temper against Rasiel and kills him, stating that "negotiations have broken down."
Xanxus' scars are present when he enters into his uncontrollable rage state of mind. He has a very specific set of flames called the flames of wrath. So, it should be no surprise when I state that Xanxus is represented by the deadly sin, ira (wrath). Wrath is described as "inordinate and uncontrolled feelings of hatred and anger. Wrath, in its purest form, presents with self-destructiveness, violence, and hate that may provoke feuds that can go on for centuries" (Seven Deadly Sins). This might explain why Xanxus can hold a grudge for almost two decades. I'd wager that he'd hold onto his slights until the day he died. In which it wouldn't matter if his father died or Sawada died, their grievances against him would still not go unpunished. Despite that the fact that they may not be around to take the punishment, Xanxus would make people suffer for it. Wrath can manifest in "impatience, revenge, and self-destructive behavior, such as drug abuse or suicide" (Seven Deadly Sins). While I would not say that Xanxus is impatient, I do certainly agree that he is on a revenge kick and that he will abuse drugs and substances (he drinks, after all). Although, in some vein the abuse of drinks is more of throwing it at people. (Too literal, I know.) However, his desire for revenge quickly runs into self-destructive behavior because he constantly picks fights with people that outmatch him. The only way to truly stop him would be to kill him, and he knows this. Xanxus plans on the fact that neither his father nor Sawada will kill him. He knows that they are both too soft; this only increases his anger and plots for revenge. He tried to kill his father twice, I'm sure that he has more plans for it. In addition, despite his minimal support of Sawada, he still loathes him. He enters into battles for another chance to fight againts him.
Let's not forget his temper at the destruction or overlooking of his personal space; hell even talking too much annoys him. Squalo takes the brunt of Xanxus' rage for the majority of things. While Lussuria tends to be a little smarter about it, he too has experienced Xanxus' abuse. Xanxus is very much a stubborn child that lashes out when it suits him. I would like to say that we're only talking about big deal issues (identity crisis with being adopted and having his hopes and dreams ripped out from beneath him), but we're also talking about Lussuria talking too much about his birthday, and the Varia bringing the wrong kind of meat after very explicit directions. We're talking about temper tantrums over said meat because it just wasn't right at that second in time despite all of Squalo's very careful planning and understanding of his boss' idiosyncrasies. We're talking that the lower members of the Varia dare not to tell Xanxus that Squalo will sort things out because it means calling Xanxus out on something. Xanxus is just has this unbridled rage that I feel is in part manifested and made worse by the fact that he's extremely spoiled. I honestly believe that pushed far enough, Xanxus would bring down the Vongola around him. I believe he would burn the whole world down around him. He tried to kill his father twice that we know of, I wouldn't put the utter destruction of the Vongola out of his reach, though I certainly think it would be a last resort or fail safe if all else went wrong. I suppose I just mean that if the Vongola got to be something he truly couldn't stand, I could see him burning it all down.