By the time we get to the end of Samson Agonistes, it's clear that we're dealing with questions of life and death. And not just literal death. Samson spends as much time describing his current life in prison as a kind of death as wishing that he could actually, literally die. Samson therefore creates a situation in which, really no matter what had happened in the poem, death is always be part of his thinking. But the fact that Samson winds up actually killing himself and many others makes ethics a big part of this larger topic of mortality: Should we be in control of our own death? What about the death of others?
Samson was a prisoner and was almost certainly going to die anyway. His choice to bring the theater down freed his people and enabled them to survive.
Samson's strength before his downfall probably made him believe he was kind of immortal, so it's about time he came down to earth and faced the reality of his own mortality.