Theo was a loyal Deatheater to Voldemort, but after they lost the battle he has recruited the Deatheaters for other tasks, with the help of his father.

You were a part of the Order and fought the remaining Deatheaters. You were one of their most powerful members

Theo knows that, that’s why he wants you. He feels he can get to you, break you. He asks the Deatheaters to abduct you. After planning they finally captured you.

The moment he saw you, his plan for you, seemed to have changed.