Regardless of how benevolent Titan seems or whoever broke the treaty first, the reason the player is dispatched by Minfilia to take Titan down is because he is a Primal period. All the Primals are a threat to Hydaelyn's existence, being aetheric sponges who absorb aether each time they're summoned. Ultimately, there is a bigger picture here than just petty border squabbles between a bunch of barely-reformed pirates and the beastmen - the whole world's future is at stake.