6. Self-Destruction Is Survival
If Annihilation has any one central theme, it is surely the notion of self-destruction, which is quite blatantly brought to light when Lena talks to Ventress about Kane "going on a suicide mission" about half-way through the movie.
Ventress insists that Lena's confusing suicide with self-destruction, adding that almost nobody commits suicide, but almost everybody self-destructs in one way or another, because of our human impulses to drink too much, smoke, throw away a good job or a healthy marriage.
The latter impulse is especially true in Lena's case, effectively annihilating her home life with Kane through her infidelity, and in somewhat poetic terms, Lena's mimic ends up doing the same thing at the end of the movie by burning its own home, the lighthouse, down.
Curiously, though, it is the drive to self-destruct which ironically also allows Lena to survive and possibly even evolve into something new.
She kills a mimic of herself in order to earn her freedom from the Shimmer, while at the same time stripping away her original cellular makeup and leaving the Shimmer a changed person both physically and emotionally.