Lizards are a widespread group of squamate reptiles, with over 6,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic as it excludes the snakes and Amphisbaenia; some lizards are more closely related to these two excluded groups than they are to other lizards. Lizards range in size from chameleons and geckos a few centimeters long to the 3 meter long Komodo dragon.
The lizard is an animal which is harmful for human beings when it comes inside their houses, and it can cause poisoning human food, that is why we have narrations recommending killing the lizard. (Sharh Al-Kaafi, by Salih al- Maazindrani, Vol 12, page 298).
It is recommended to wash the whole hand after killing the lizard, but Ghusl is not obligatory for that.