杀掉(shā diào) - to kill off, to kill in Mandarin

杀掉 (shā diào) is to kill and be done with it. The 掉 on the end is a resultative complement, the same one in 扔掉 (throw away) and 忘掉 (forget), and it says the object has been got rid of rather than merely acted upon.

That disposal flavour is why it fits animals, pests, bacteria and, in fiction, characters written out of a script. It carries a shade of practicality that its neighbours do not: 杀死 states the outcome plainly, 杀害 is the grave word used of murder victims in news reports, and 杀掉 sounds like a decision someone made.

The trap is thinking the complement is optional decoration. Bare 杀 needs an object and a context to feel finished, so 把它杀了 or 把它杀掉 is what a speaker actually says. Note the 把 construction in most of the examples above: with a resultative like this, the object usually moves in front of the verb.