>>534945696
My guess is that Chara was chosen due to the artist utilizing Chara (Demon) and they fit the dark/bloody angel motifs of Zero 2 more than Flowey (another way to consider is that Chara's sin would be Wrath, Flowey's Pride). Not the most versed to the boss though, I know them via other videos like a MotI song but not played Crystal Shards.

So, Chara posess Frisk during the genocide because he likes Frisk and cannot allow the player to corrupt him with violence? What other reason could there be for posession?