Tarasque, when Lysandre was younger, was notably more calm than other gyarados. Mostly due to the fact that Lysandre raised him with plenty of encouragement and effort, but also because Lysandre himself was softer back then, too.
Respect, care, and attention all go a long way to preventing the typical rages of newly-evolved gyarados — there was some energy in those first few weeks that needed to be dealt with, given an outlet, but once he calmed down, he stayed that way for a long time. Affectionate and aware of his size, he would let children (such as Paxton or Corbeau) climb on him, would carry them without a fuss. Kind strangers and other researchers were allowed close, and he was often used as an example model of a healthy and well cared for gyarados.
However. Pokémon have a tendency to be very similar to their trainers in many ways, and when Lysandre started spiraling, so did Tarasque. Where normally he would stay back when watching people have conversations with Lysandre, he turned instead to curling around him protectively, growling and snapping if people questioned his trainer. He was quite the intimidation tactic used against Flare members who dared to speak up against Lysandre's ideas, or those who seemed likely to allow word of the work going on to slip outside of the team.
Mega evolution made that worse — upon mega evolving for the first time, he nearly took a wall out of the lab. It took Lysandre a considerable effort to ensure he didn't harm anyone nearby, and his aggression didn't calm until the mega evolution wore off. Even so, he was different afterward, almost as if he craved undergoing the transformation again, hungry for power.
With L, Tarasque has calmed considerably, seeming regretful of his actions. His memory was not fragmented, he remembers everything he and Lysandre did. His aggression has turned into a mourning of the things they shared before, and instead of growling and lashing out, he's more likely to be heard making quiet, comforting rumbles when L seems lost.
