Why can't you burn down cities you don't want/need?

[Martin]

The happiness system is one thing but this.. it wrecks the game complete. It's a completely waste of time.

[Kettlebloke]

You can 'raze' cities when you take them over unless it is a Civs capital or a holy city.

[DemonSlyr007]

if you are referring to the permanent war weariness... it actually isn't permanent. It is only permanent for that era. Those debuffs will go away at the start of your next era. Raze away if you think it will be your last war of the era, or deal with the debuffs for being a complete jerk razing cities to the ground.