None of the three great clans had openly killed any of the descendants from the others. But it was false to say that they hadn't killed any descendants.
A lot of the descendants from the three great clans would vanish without a trace once in a while. It was hard to pinpoint how they died, but most of the victims were talented descendants.
With that thought, it wasn't hard for Lawrend to realize what the elder from the Optim Clan was trying to do. With his high standing and age, the Heaven Mage elder should've realized the connection of the Rubrignis Clan with the Ignis Clan.
Lawrend didn't research about it, but he could imagine the Rubrignis Clan appeared because a descendant from the Ignis Clan moved to the Undrasil Kingdom.
His talent was like a bright sun in a dark space. He unknowingly attracted the attention of one of the three great clans. That elder must've realized his threat to the balance of the three great clans early on.