Although Obito was also a pitiful wretch, that did not stop Shamu from hating him.
He had been young, brainwashed by Madara, and had completely abandoned the former self who cherished his companions, who was kind, caring, optimistic, and lively.
Rin died, and he wanted the entire Shinobi World buried with her.
When he needed the Rinnegan's power, he stopped Kyūsuke Daibutsu from rescuing Yahiko and drove Nagato into darkness.
When he needed to weaken Konoha and reclaim the Nine-Tails, he released the Nine-Tails while Kushina was giving birth, when the seal was at its weakest.
He had no regard whatsoever for his teacher and teacher's wife, and what had Konoha's civilians done to deserve it?
When he needed money and talent, he used the Sharingan to control the Fourth Mizukage, Karatachi Yagura, and imposed tyranny upon the Hidden Mist Village.
While amassing wealth, he eliminated everyone who threatened his plans, as well as anyone who could threaten the Fourth Mizukage's position. Countless bloodline clans in the Land of Water were wiped out as a result.
As for how many people died in the Fourth Shinobi World War, there was no need to mention it. Countless families were torn apart because of it.
Obito's crimes were simply too numerous to recount.
Although Obito had also been Madara's pawn, and another pawn would have appeared without him, that did not excuse him.
Kakashi's mercy in the Kamui Dimension greatly irritated Shamu. What an ungrateful wretch—he had even forgotten the deaths of his teacher and teacher's wife.
How many Mangekyō Sharingan had Madara accumulated over the years, and how many did he still have left for Obito?
Every extra chance to kill Obito and destroy one of his Sharingan was a wonderful thing.
The more trouble he could cause Obito, the better