Zeeskeit, you have selective memory.
You seem to remember all the fire brimstone, but not the Gemara that a court that executes once in 70 years is considered “bloody”.
There is an argument whether there ever was an actual “condemned city”. The Torah is promising us that in the rare (if ever) case of an Ir Hanedachas (condemned city), that the acts of execution will not undermine our natural sense of mercy.