MC didn't go berserker at all in the novel, it seems like he didn't even cross his siblings there, but when he faced them in the school battle he was calm.
So is a mistake of the manga, and I call it a mistake because it made it look like he was prone to lose himself in his rage, his power is literally from the mind and he trained for 2000 years for not going against his own word, he losing it for that was adamage to the character we know.
Note for the future, no one believes his skill is telekinesis because is largely know that is a "miss" skill, it can only attract objects to the user, soon skill evolution/level will be introduced in the plot, it means when a user of a skill breaks through the next level and is another reason no one believes MC and think he is bluffing to keep it hidden, as knowing a skill would make someone able to create a counter to it, anyway, the max skill level for telekinesis is 10, is a know stop on it, is not that all users reached that and couldn't progress, is the level cap it have, hence why people can't believe it, most likely the goddess removed that limit, that is not giving a cheat but removing a handicap.
MC gets his skill appraised later out of curiosity to know the level, it makes the appraiser faint with a nosebleed, is a string of 999999999- that doesn't end.