[chasesinister]

My take on why Tsurugi had to be killed is that as a Worm he has killed at least two people - the real Tsurugi and his sister. While Hiyori and Tadokoro as far as we know never hurt anybody. So Tsurugi was more or less sentenced to death for murder like all the other Worms that killed somebody. But that's just my take on the matter.