the flower is kind of a souless being with Asriel memories: since monsters turn into dust upon death, once that dust covers something, it fills that thing with the esence of the monster. But flowey is aware of him being nothing but a vessel of what once was the real Asriel (Also it doesn't have a soul of his own, therefore it lacked the skill to feel anything) which only leaves the terrible experiences Asriel had, leaving to the point where nothing is important and you have to kill or be killed in this world from his perspective
Spoilers He pops up int he beginning in the game telling you to run into the white pellets. They do damage but he tells you they're good. He gets pushed away until the last part, where you're fighting the King of the Monsters. After weaking him, you can spare him. The king after that, the king talks about how he will take care of you and some really happy good feel stuff until flowey kills him right when he's finished talking, takes his soul, and the other human souls to become a demonic god that you have to fight
he's pure because he doesn't understand what he's doing is wrong. He just acts on what he know's or how he believes things are. Which is why he doesn't understand kindness.
Neeyigga you high? He understands fully what he does! He just goes mad from living life over and over and over on the resets, and never truly feeling anything. He went through most likely, hundreds of lifetimes, each extremely similar to the last.
Literally this. He literally has no soul because hes the reincarnation of a boy who wanted things to go back to the way things were before his friend died, and when he was reincarnated, he didn't have a soul and had no capability for compassion for other beings, leading to him being a total ******* asshole. This leads to him either "playing" with you, in place of his friend, for the rest of eternity (until you beat him, which he doesn't think is possible), him becoming a god, taking control of the reset button (the actual reset button to reset your save file) so it resets to when his story started and he can be happy again, or he tries to stop you from killing everything and everyone because you're more evil than him and if you kill everything he cant live out his plan.
No its actually the character's theme chara or whatever. because a megalomaniac is "a person who is obsessed with their own power."
That doesn't sound like sans to me.
Pic related, that quote is also what leads me to believe it's not sans.