1) Orochimaru
-His parents were killed at a very early age as he witnessed others he cared for die left and right in war. This was a bad mix with his genius and fear of death and being forgotten, thus he began to covet immortality and all jutsus, not only make himself last physically, but also figuratively. Originally he wished to find a way to keep people from dying, but over time he became selfish and deranged and only cared for saving his own life.
2) Sasuke
-Sasuke witnessed the death of his parents and clan by his own brother no less at the age of seven before he had even emotionally matured. He did not possess Itachi's experiences or objectivity and thus could not handle it. As a consequence, it became his drive and thus a part of him. Cutting his bonds with his friends and closing off his emotions for three years was a bad mix with learning the truth about Itachi and Danzou and Konoha shinobi mocking the uchiha and it threw him over the edge.
3) Madara
-Madara lost all of his brothers and felt subjugated by the Senju his whole life. This, combined with the ultimate knowledge he obtained from the Uchiha tablet drove him over the edge.
4) Obito
-Obito was badly injured and left bedridden with Madara slowly brainwashing him over to his ideals. This combined with Rin's morbid manipulation by the blood mist ninja and death by his own comrade, both still children no less drove him over the edge.
5) Kabuto
-Kabuto was manipulated by Danzou into killing his foster mother, who had been brainwashed into killing him. That's one of the most effed up villain backstories in the manga for me. Orochimaru inspired him with his knowledge, experiences and ideals and promised him justice and revenge, thus he idolized and worshipped him to the point of leaving behind his identity, which he lost value of and becoming him after he was sealed.
6) Nagato
-Witnessed the death of his parents, lost comrades and his best friend to a treacherous tyrant after he had spent most of his life homeless, starving and being hunted.
7) Kisame
-forced to kill his own comrades
8) Zabuza
-forced to kill his own comrades as a kid.
9) Neji
-father committed suicide to protect his uncle and both he and his father were subjugated by uncle.
10) Gaara
-his father treated him like a monster, put a bijuu in him, neglected and tried to kill him several times, his uncle tried to kill him and he was forced to kill him, led to believe his mother hated him and he was subjugated by the village.
11) Kimimaro
-imprisoned and weaponized as a child by his own clan. Never possessed freedom or the ability to think for himself.
The one I believe had the least motive was Danzou. He talked about how he wanted to preserve peace, security and unite the world, but the only motivation I ever saw behind his antagonism was his disdain for Hiruzen being kind and peaceful as well as chosen to become Hokage over him.
