TC abilities are only active when the character is in territory (or battle obviously).
If a TC character is removed from territory (discarded, banished, returned to hand), it's ability simply turns off even if the removing ability did not also negate the character.
In this scenario, Samson enters battle and completes his ability. Since Leviathan has been removed from territory, his ability turns off and he would not trigger by Behemoth blocking (nor would he trigger once Behemoth negates Samson).