Why does George say that they cannot let Lennie escape to live on his own ?

George says that if Lennie were on his own he would starve.

George was a long time in answering. "Guess . . . . we gotta tell the . . . . guys. I guess we gotta get ‘im an’ lock ‘im up. We can’t let ‘im get away. Why, the poor bastard’d starve." And he tried to reassure himself. "Maybe they’ll lock ‘im up an’ be nice to ‘im."