What is the ruling on killing large lizards (like the monitor lizard), and is the one who does so rewarded?

Whatever harm is proven from lizards and other animals, it should be killed to stop its harm. As for what is not proven to be harmful, it should not be killed, because it was only created for a wisdom willed by Allah, and among these is maintaining ecological balance. The monitor lizard is known for preying on snakes and vipers, so killing it and eliminating it in an environment will strengthen the spread of snakes there. However, if harmful types of monitor lizards are found, or some individuals deviate from their peaceful nature and inflict harm upon humans or their property, then it is permissible to kill them to repel their harm.