Oh, wait, you forgot these were the Israelites we're talking about? Yeah, turns out they didn't have the heart to make a clean sweep of the Canaanites. Especially when it came to valuable livestock, or the more beautiful Canaanite women, or the booze, or... well, you get the idea. They were seduced by the booty. Besides that, they seem to have lost a bit of their focus. They chased out the evil until it was what they, in their human estimation, thought was 'far enough', rather than going all the way.
So what happened next?
If you guessed "They had to deal with the Canaanites again later," then you win a kewpie doll. Bonus points if you added "They were corrupted by their involvement with the vestiges of Canaanite culture."
The Israelites just didn't have it in them to completely wipe out the evil. They wanted to keep a little of its riches for themselves, and they had some glimmerings of misplaced mercy.
How can a Christian say that mercy is misplaced? Aren't we supposed to forgive and love everyone? Even and especially our enemies?
This would be where the Batman analogy comes in. Obviously I don't support genocide. If God gives a direct order, then it's to be followed, but anyone else declaring a whole people group to be "evil" I'd look askance at. But take the whole thing as a morality tale, the way it should be used to teach.
How many times has Batman beaten the Joker, over the years? Or Two-Face, Scarecrow, the whole lot of them? But especially Joker. Dozens? Hundreds? And how many people get murdered, tortured, maimed, driven insane, or otherwise harmed severely when the Joker gets loose (because he always does). Thousands, according to the comics.
Batman refuses to kill. He plays catch and release with the Joker, and every time it costs someone their life. He doesn't want blood on his hands, so some mother loses her son, and the tragedy is repeated hundreds upon hundreds of times.
He'll go out and fight the evil. He lacks nothing for courage, skill, or dedication. He'll fight the evil until he draws his last breath, let nothing take away from that. But he won't kill it. He simply won't go over that threshold and take it as far as it needs to go.
It costs him in effectiveness, too. Criminals lose their fear of the bat when they know he won't take their lives. For some of them, that's the only thing they hold dear. His justice has no teeth.
Consider then, how your enemy views you, when you won't eliminate the evil from your life. And make no mistake, you have an enemy--we all have the same one. The tempter, the betrayer, the liar.
Somewhere in your life, you give him ground. I catch myself doing it, too. I'm not okay, and neither are you, and neither is anyone else. Maybe you just can't let go of a grudge. Maybe you quietly envy another. Perhaps you have a stash of Playboys under the bed. Or "hentai doesn't count" or somesuch nonsense. You're stingy with money that you could easily afford to share with the poor. You justify it somehow or another.
You think that this sin, or that one, is "okay." You hold back a little for yourself. Somewhere in your life there is a line that you won't cross, even though God orders you to. Whatever is on the other side of that line, that's your golden idol. That's your Canaanite mistress. That's your sympathy for the devil.
I'm sure he appreciates your mercy, but there's nothing Christian about it.