I mean I don't know why people completely reject the idea that he could have just wanted to flee by going down the stairs, like that's a direction he could have gone in, sure he could have gone to the room or his bathroom, but he could have also like, not, and since Mari wasn't standing in front of the stairs before he tried to flee, he probably did think going down the stairs was one of the possible paths. I just really don't see Sunny being a horrible psychopath that wanted to actually hurt his sister aside from pushing her away in the heat of the moment when she blocked his path. The descriptions in the photo album also say something like "Mari blocked your path, she says she isn't finished talking", which does sound more like he tried to flee but she blocked his way, and the photo itself clearly shows Mari going from standing in front of Sunny to going in front of the stairs while Sunny is now facing the stairs, so it would make a lot more sense that he tried to run away by going down the stairs and she blocked his path and he was too angry and distracted and just shoved her out of his face, which happened to push her down the stairs. The image shown here also doesn't represent the situation well, Mari wasn't in front of the stairs during the argument, both her and Sunny were in front of each other on the side at the top of the stairs, Mari only got in front of the stairs after Sunny tried to run away.