What are the legal consequences if your landlord is unwilling to return your deposit upon vacating with no damage which causes you to then deface the outside wall writing the landlord is evil?

Where are you located country wise?

UK it’s illegal for them not to hold it in a deposit protection scheme and you can go debate the charges the landlord is trying to put on you through the protection scheme

These schemes are usually pretty good if you have evidence or the charges are overblown at getting you your deposit back

If you start defacing the property like you suggest though you can kiss the whole thing Goodbye