Weird... You're complaining about dishonest Christians and then do what they do, present biblical evidence without context. Leviticus is a book of laws. You're basically angry about a legal code.

It's Psalm 137:9, not 137:4; in a chapter talking about the Jeiwsh experience in Babylon. And the quote is about vengeance for what Jews went through in Babylon as slaves.

1 Samuel 15:3, also about vengeance for what Amalek had done.

And so on so forth...