Curate, connect, and discover
I'm sure this has been said before, but I really like the idea that Tadius fantasized about killing the king (and probably the prince, too) before he met Ella, in the same way Ella fantasized about killing her step sisters and mother. They both feel trapped, aidless, abused, and lack a large level of autonomy; which is why both of them are eager to enact their revenge the second they have the ability/opportunity to do so.
That's why Tadius is so quick to:
1. Believe Ella is talking about the king in her monologue (she's hypothetically saying what he's secretly been thinking for a long time)
2. Smother the king with a pillow (both enact unapologetic (/pos) vengeance before their exact plan is fully realized because they are so eager to gain both freedom and justice)