Really comes down to what you perceive as your "enemy".
It could be singing about human's flaws ... greed, hate, selfishness, pride, ego etc. That is these characteristics that humans have and will always have, and thus inevitably these will lead to war, conflict, chaos in the world. So although humans are mortal, these flaws are immortal.
It could also have religious connotations. "Evil" is always there tempting humanity, is God / religion your only "solution"?
Or could be something more personal (although doesn't seem as likely given some of the lines). In this circumstance the "enemy" is more your internal demons, such as depression, regret, doubt etc. etc. These "demons" you may defeat but they can re-surface anytime and it is an ongoing internal struggle to defeat them (keep them at bay) for a lifetime ... they can not be "killed".
Really comes down to what you perceive as your "enemy".
It could be singing about human's flaws ... greed, hate, selfishness, pride, ego etc. That is these characteristics that humans have and will always have, and thus inevitably these will lead to war, conflict, chaos in the world. So although humans are mortal, these flaws are immortal.
It could also have religious connotations. "Evil" is always there tempting humanity, is God / religion your only "solution"?
Or could be something more personal (although doesn't seem as likely given some of the lines). In this circumstance the "enemy" is more your internal demons, such as depression, regret, doubt etc. etc. These "demons" you may defeat but they can re-surface anytime and it is an ongoing internal struggle to defeat them (keep them at bay) for a lifetime ... they can not be "killed".