Chained by Sand

Gaara is an interesting character, in that he started out very much unlike most other villains in the Narutoverse. He was not desirous of omnipotence, as were Orochimaru, and various Akatsuki members. He was not forcibly subverted to the will of a powerful evil being, like the Sound ninjas are. Rather, he was corrupted and was trapped within himself, seeking only the meaning to his existence by erasing other people's. He was not under the service of "true" evil, though he was the vessel for the Shukaku, from whose influence he could barely escape and, having fallen to despair a long time ago, no longer had a will to try. Sounds like quite a sad predicament.

And it was.

It only becomes clear that he was once a darling of a kid and had already been hit hard by the cruelties of life at the time we get to his flashbacks and watch about his background there; the first time we meet him, he was already a psychopathic murderer and this is when he was at his lowest point.

Along the way, we eventually get to learn of Gaara's hopes and fears, and come to understand the motives behind his actions, from his interaction with other characters, most notably Naruto, Sasuke, Yashamaru and (although less so) Shikamaru and Rock Lee. We begin to realize that Gaara was once a good and high-borne child capable of great deeds and small kindnesses. He was a character pitted against a situation in which he was doomed to fail and fall low. From a very young age, he had his life fall into relative darkness compared to what it could have been if only he was allowed to live a normal life; his very existence as a Jinchuuriki was a source of pain to him; his future seemed as bleak as an eternal trudge over dead, barren land. We learn of his frustrations at being trapped within a role to which he, before he was even born, has been relegated. We see his struggle to be as much a part of the community as everyone else, and his pain at everyone's rejection of his efforts. We see a character who started out with the most worthy of intentions for his fellow villagers, but who, through a cruel set of events, came around to thinking that there was no hope. We see child who was blamed for his mother's death, and whose other acquaintances was a source of pain and disappointment to him except for one man. But even then, there might have been some hope in the said man, until he too dropped his mask.

It's enough to make anyone touchy. And no one can go through all of that with perfect composure and zero mistakes. Even without agreeing with the egotistical principle he adopted for himself later on in life, we can at least understand the state of despair that drove him to the terrible deeds by which he took other people's lives in order to justify his own. Rather than seeing him as simply a "bad guy" for his madness, we come to understand how he came to that point, burdened by grief that he could not reconcile, along the way hurting not just other people but also himself.

His despair was heavily influenced by the betrayal of the only person who showed him kindness (as he saw it), factors played upon by powers far greater than him. Gaara probably saw his uncle Yashamaru as a symbol of "love"...a healing for himself, so when that hope is snatched away, all slivers of sanity came with it.

How did Yashamaru work on Gaara during his very last moments?

He certainly made great use of negative talk, influencing the kid through looking on the doubtful, pessimistic side of everything. Here was the man who supposedly showed him love, but now eminating hatred and crushing all hope with his every word. Chibby Gaara was actually unable to move for the rest of the time. Imagine being completely shocked, kneeling on the ground, tears flowing like rivers, watching bloody images of suffering and hate, unable to avert his ears and being faced with a presence powerful beyond his understanding that desired to turn his mind to darkness.

That to Gaara's small, pitiful, trusting mind was, put bluntly, a horrible betrayal to his thirsting soul. Between that shadow of fear, the weakening of his will, Yashamaru's meddling and bad times, Gaara was subdued and has been left with little choice. Being at such a young age, there was no way he could run out of Sunagakure, nor could he talk his father or his assassins out of their attempts. Gaara sought nothing else but love and acceptance from his family and fellow villagers, and he had none, nor any chance of ever gaining any. From anyone. Yashamaru sure as heck made certain that Gaara understood that. He is at the bottom of the social heap and so has little to lose when he acts out of a sense of vengeance. He is certainly brought low, and he ends up with quite the opposite