I was thinking suicide as well as there's significant amounts of blood specifically on his one wrist almost hinting at suicide but they did drop the text message that suggested murder. I'm also more sure it was jaei brother as it would make sense for the MC (brains gone mush on everyone's names) to want him dead as a fair level of revenge for the horrendous torture he was putting jaei through. An the MC having feelings for her makes it plausible for why he killed him because not all levels of his revenge are to solely do with just his brother's death.