Of course people are sympathetic toward Akechi... He's a tragic character. He doesn't know what it means to be able to rely on other people. Akechi even saves you from cognitive Akechi at the end and genuinely places his trust in protag to expose Shido. That being said, I completely agree with you 100% that his actions are unjustifiable and he IS in fact responsible for all of his actions. But the thing is, forgiving someone and feeling sympathy are two different things. I thought the rest of the Phantom Thieves were out of their goddamn minds when they were suggesting Akechi join. I don't condone his actions, but it's not hard to feel pity for the guy given his backstory, you feel me? Even if he is responsible for many deaths.
