He killed Ragnar because he was the leader of bb clan. He knew Sonja was capable of leading the bb clan to a war but not capable to lead them. So he assassinated Ragnar and framed a trade competitor of his. Two birds with one stone. After Ragnar, he could influence bb clan even make them to join his clan in long term by influencing new leader. That's why he heavily invested in warriors and send Astrja to show he got revenge of Ragnar, therefore respect of bb clan. Also by eliminating trade competitor he was going to be rich enough to unite all the clans and move towards bigger goals. Ragnar was good warrior but poor leader, he was an unruly oaf who cannot be controlled or tamed, so getting rid of him was crucial.
Candrid's chalice scene was important at this front. It was funny but gave an important information. Everyone was seeing Candrid as greedy, yet stupid one. So, I think Torstein saw her perfect candidate. She had no loyalty to Ragnar, stupid enough to betray him and greedy enough to be sway by wealth which he had plenty. But Candrid was none of them and played him.
Candrid is playing the long game. If you think about it Torstein, Nox and Candrid have the same goalm uniting all the clans under their leadership. She knew it was near impossible to overthrow Nox so she took her people. Brought outcast to her side. Now, if she can make Mathilda the new leader of the ww clan, she practically manage to unite all the clans.
Probably Draugs are can be controlled by magic. If I had to guess, Candrid figured it out how. Also, another guess of mine, like the Sandstorm evil gods are helping her. After the last update, maybe Astrja's step-father helping her.