Unless something has made King Amazing's ability CBN or CBP, his ability will be negated by Tower of Thebez from the beginning, which means he can't choose an evil king to block at all.
Actually no, in the situation described Amaziah is not negated before battle.
Player A has a Canaanite in tower and an evil King of Israel in territory, so tower is not active. He makes a rescue attempt and using Amaziah's CTB ability selects the evil king to block. Tower's conditions are now met because the evil king is no longer under his control, so Amaziah is negated.
However pre-block choose the blocker is inherently Cannot be Interrupted, so the evil King stays in battle.