In Immortals, prior to the events of the movies, the gods (i.e. Zeus and friends) fought against the Titans and won. The surviving Titans were then imprisoned. Why not kill them all, as they are still a threat to the gods? It just seemed like really bad planning to leave them alive and imprisoned, expecting them to never be a problem again.
I haven't seen the movie, but speaking from a military/tactical POV, I can say that winning a battle, isn't the same thing as dominating. It's possible that if one side of a conflict is beaten enough, that they will accept surrender terms - but not annihilation. If the winner attempts to impose total destruction, it means their opponents will now fight to the death, possibly causing more casualties on the winner's side also.
Titans, like Gods, are immortal. This not only means that they do not age, but also that they cannot be killed. While some fade in power and into obscurity, others are punished eternally with various tortures (see: Prometheus).
I'm not certain if this is addressed in the movie Immortals, but in the original mythology, the Titans were the progenitors of the Olympian gods.
Zeus, and many of the other original Olympian gods/goddesses, was the child of Cronos and Rhea. Cronos had made a habit of eating his children when they were born to prevent them from growing up to become threats to his power, so there wasn't much familial obligation there, but they may have spared the Titans simply because they were family.
This is dealt with in the film's opening voiceover
VO (ZEUS): While the vanquished were renamed Titans and forever imprisoned within the bowels of Mount Tartarus.
So there you have it. There was no need for the Gods to slaughter those they'd vanquished since they were effectively dead anyway, buried underground for what they assumed would be the rest of eternity since there's absolutely positively no weapon on Earth powerful enough to release them.
Actually there is a confusion stated here. Chronos is actually a primordial diety as with gaia in Greek mythology who bore the first generation of titan's (12 of them). Those 12 also bore the second generation of 12 that over threw them. Then birthed some of what is referred as Olympian gods zeus being the youngest (some would reckon oldest as he had to free his brothers from Chronus stomach as they had already been eaten).
However back to point in hand 'Chronos' is basically considered father time and creator of the universe, gaia mother earth and a few other's that were primordial dieties. 'Chronus' was the cannibalistic father titan of zeus.
Notice how 12 is referenced throughout most religions and myths. 2 sets of 12 titans, 12 Olympian gods, 12 disciples, 12 zodiacs, ect. Also death and resurrection, as dyonisus was killed as an infant, zeus sewed him into his thigh and he was reborn. But in Greek mythology gods and titans don't die, they fade when power of their domain is lost or taken control of hence being able to be overthrown. All of which was by the hands of their offspring which is more likely metaphor for giving way to younger generation's.