The narrator of "The Man He Killed" seems to believe that many people enlist because

they need money to live

they believe dying for one's country is admirable

they support a cause and want to fight for it

they get drafted and have no choice

They need money to live

Explanation:

From Stanza four of the poem, "the man he killed", when the speaker could not find any reason to have killed the man he went into battle with, he justified himself by saying he enlisted in the army because he could not find any meaningful thing to lay his hands on. He said he joined because he knew nothing else to do and joining the military was a last resort.

"He thought he'd 'list, perhaps,

Off-hand like - just as I -

Was out of work - had sold his traps -

No other reason why."