Theo developed greatly throughout the movie, whether it be through emotional turmoil or catastrophic experience he went from a mentally and physically numb character to a man who would do anything to protect a baby that wasn’t even his.
The writer accomplishes this by establishing his initial numbness, ex: Baby Diego dies and he doesn’t seem to care, smokes and drinks throughout the movie, has little attachment to anyone besides Jasper, an old man in the woods who even he thinks is crazy.
Then as he is stripped down to almost nothing, and the people he loves most are taken away from him. His ex who hints at a future with him is killed within the first 30 minutes of the film and that is the first step of a long process which turns our once numb character into a vulnerable savior. Theo then loses Jasper in an attempt to free Kee from her many pursuers. This tears him apart and leaves him essentially grasping for any emotional stability he can, thus he latches on to Kee even more.
It became obvious that Theo was going to die before the movie ended when he had helped Kee birth the baby. It was then and there that he had fully opened himself to the world and had completed his growth as the vulnerable character. Then sadly, as cinema has proven time and time again, when a character completes their development, they either die, or begin a new life. In a dystopian London it is a pretty obvious choice for the writer to make.