Life-and-Death
His brothers were obsessed with him, and that had been fine for a while. Eren thought it was obvious to desire vengeance. They might not even know what their parents did to Eren, as they had been too young to remember anything. Or, if they knew what happened, they might consider it the only feasible way to salvage Eren.
His brothers wouldn't be the first ones to believe that Curses were impossible to control and that all Curses would transform you into a monster. While it was certainly possible for a Curse to transform you into a monstrosity if you allowed it to consume you whole, that rarely happened under the Nest's supervision.