I was re-watching Naruto, and when Gaara's backstory was being revealed, I noticed something that I never really understood earlier. Yashamaru was ordered to kill Gaara, yes, but in his last breath, he lies to Gaara that he never loved him, and that he couldn't forgive him for killing his sister during childbirth.
I read through the Naruto wiki - https://naruto.fandom.com/wiki/Yashamaru#Personality - to confirm whether he was actually lying to Gaara ,or whether my memory deceives me, but yes, the wiki also confirms that Yashamaru "claims" to hate Gaara but in reality, hated the Fourth Kazekage. So the question still remains - why did he lie to Gaara anyway? He was the only reason why Gaara was able to stay relatively sane for so long, and he was Gaara's only glimmer of hope and happiness in an otherwise lonely environment. What good can possibly come of stripping Gaara of that sliver of happiness and love?
My theory is that because Yashamaru hated the Fourth Kazekage, maybe he wanted Gaara to truly lose all semblance of humanity and fully embrace his inner demon and destroy the village. Maybe as a way to avenge his sister - if the Fourth Kazekage could do such horrible things to "protect the village", then enabling his own son to destroy his precious village would be a big middle finger in his face. Sounds like a dark theory, but I couldn't come up with anything better.
Does anyone know any canon reason? Or a better psychological explanation?