What is a word that could define someone who likes to cause conflict?

What would you call someone who does things knowing specifically that his/her actions will cause pain and/or conflict or completes an action just to get someone in trouble or hurt them?

For example, in Private Peaceful there are two people that are in love, but the Colonel goes out of his way to tell the father of the girl that the boy is a thief and would be bad for their reputation.

What is a word that would describe him?

Solution 1:

An excellent word is malefactor, which is basically Latin for wrongdoer. And much like a benefactor acts benevolently to others, often anonymously or indirectly, the same is generally true for a malefactor acting malevolently - like in your Colonel's scheming*.

A phrase that frequently comes up in describing these characters is bad actor.

*See also the most famous malefactor of all time, Shakespeare's Iago.

Solution 2:

Such a person might be described a a trouble-maker.