Teen Wolf, Season 5

Valack:
[about Theo and his sister] Why did he let her die? What did he want?

Lydia:
Her heart... He wanted her heart.

Valack:
There's no record of a heart condition... But a transplant would make Theo a genetic chimera like the others. The hypothermic conditions would keep the organ viable. But why him? Why choose a ten-year-old willing to kill his own sister?

Lydia:
[weakly] Why do you care?

Valack:
Because Theo was the first step in the right direction for the Dread Doctors. Think of who they are. Parascientists who have prolonged their lives and twisted the laws of Nature in pursuit of one goal-- the creation of the perfect killer.

Lydia:
The last Chimera...

Valack:
And now they've done it.

Lydia:
But Theo doesn't know who the success is.

Valack:
It doesn't matter. There's a reason they chose him, a reason he failed, a reason they changed their methods, and a reason the finally succeeded. It all starts with Theo.