If you do not eliminate the enemy completely, they will come back to fight you and you may get caught unaware.
God’s idea about the enemy is that they must be completely wiped out. If the enemy is not eradicated, even if it is just one, it would ensnare you and be a problem.
Jdg 2:2-3 NET 2 but you must not make an agreement with the people who live in this land. You should tear down the altars where they worship.’ But you have disobeyed me. Why would you do such a thing? 3 At that time I also warned you, ‘If you disobey, I will not drive out the Canaanites before you. They will ensnare you and their gods will lure you away.’”
“Resist the devil” could also suggest that you get rid of him so he does not hinder or stop your progress.
The enemy target your weak areas. So, you need to be on guard for any onslaught. This is why you must resist evil and eradicate it completely, if possible.
The enemy you spare today will come back to hurt you.