Xanxus has had the ability to use the flames of wrath since he was a child. This always amazes me because it means that he was able to channel his flame with no issue. Was it wrathful as a child? I can't say for certain. However, we do know that he lived in poverty with his mother and that he believed the Ninth was his father. Could he have had some anger towards a man that let his mother suffer? Possibly. On the other hand, it was after his mother saw the flames that she began to delude herself and her child into believing that Timoteo was his father. After meeting with the Ninth, Xanxus became part of the Vongola household. Timoteo took him in and raised him as his own son. Here's the issue, the Vongola family is based very much on capabilities of bloodline. One must be of Vongola blood to inherit the family. Despite the fact that Xanxus was raised as a Vongola, the truth is that he has no blood ties to the family. Everything he had wanted, everything he had been raised to believe was his birthright slipped from his hands at the age of sixteen.
Timoteo couldn't allow the child to suffer on the streets, but he also couldn't bring himself to tell the mother or the child that he was not the father. Instead, he confided in a journal that the child would later find. This revelation destroyed what value Xanxus had for the world. He'd grown up believing that he would inherit the family, and that he was a candidate as much as any one of his brothers. He mistook the Ninth's kindness for betrayal. Frustrated and unable to cope, he decided that killing his father and taking over the family would be the better option. It was the only option. I have always found this particular event a double edged sword. They call it the cradle affair and basically, it breaks down to Xanxus trying to kill his old man because he feels betrayed for having been lied too.
I feel like the hardest part for me is the realization that Xanxus understands his mother was completely delusional. She had convinced herself of something that had never happened. Then, the Ninth felt sorry for him and took him in. No one feels sorry for Xanxus, he's too proud. That pride ultimately led to wrath, I think, at least in so far as he could internalize. I imagine, also, that he was handed almost everything he could have ever thought he wanted that when he legitimately wanted something and couldn't have him it slammed against him. Why? Why was he an inferior choice? Why did people pity him? Why had the only father he'd ever known lie to him? He started to feel like he had only ever been a nuisance or troublesome, some secret that had to be kept hidden. Something unwanted. His mother got rid of him, eventually the Ninth would too.
Xanxus refused to let that happen. Instead, he'd kill the old man and take what he wanted. He was used to taking what he wanted. He was raised in the mafia for crying out loud. It was nothing to kill a man or destroy something for the sake of power. That's the general idea that he was raised on. He felt betrayed and it angered him that someone had the audacity to betray him. I believe that Xanxus trusts no one; I believe that while Timoteo was trying to be kind, he really did hurt Xanxus. I believe that Xanxus is just a spoiled brat that doesn't know how to cope with life. Let's face it he's had everything handed to him. The idea that he was adopted and no way related to the Vongola shook him to his core. With that thought, what else was the Ninth hiding from him? I sympathize because the life he adored and wanted had been based on a lie. I sympathize because the one person he thought he could trust had fed him the biggest lie of his life. I sympathize because he didn't even find out from his father, but from his father's writings and I doubt that an actual dialogue would have been any easier to take. On the other hand, I also understand that Xanxus was just mean spirited. He believed that he was a gift to people. He believes he's superior to everyone. This can stem from two things, one that he is superior or two that he's actually pretty insecure. Either way, after finding out that he was adopted just caused him to stay in a perpetual rage in which he lashed out at everyone.
Ultimately, Timoteo defeats him with a move called the Zero Point Breakthrough. It has the ability to seal dying will flames and is meant as a counter measure to power. It blocks all light and positivity. It freezes. This is how Xanxus gets his scars. Xanxus spends eight years frozen. When he awakens, kills three of the other candidates for the tenth boss position before sending his men to collect the Vongola Family Rings. This event, this cradle affair, is ultimately what leads Xanxus into the antagonist's role.