For example: they say bad things about you or make you mess up on a project so their's looks better. Almost like a bully, but not quite.
Someone who says bad things about you could be a "backbiter" or a "backstabber"
Someone who makes you mess up could be a "saboteur".
Someone who does unethical things to promote their career could be a "neck-stepper".
"a person who destroys or damages something deliberately : a person who performs sabotage"