When we are shown how Roan "cleans" bodies. The process is a pretty terrifying thing to witness, as is demonstrated by Alex quickly vomiting after witnessing it.
Especially when Nikita herself is being prepped to be disposed of in such a manner in the first season finale, and can only watch, paralysed, as Roan is about to make her easily decomposable via a freaking huge needle.
The poison that Nikita is given in "The Phoenix" by Ari that will literally liquefy her stomach.
Not to mention how Nikita kills the spy pretending to be a mother in the same episode.
Owen describing how cleaner equipment works.
Hydrochloric acid. Carbon steel bone saw. That's a blood siphon. Bodies fold better when their fluids have been drained.
Amanda having to "trick" Alex's kill chip so that she can escape Division without having it kill her. The "trick" is she kills Alex with a large electric shock and restarts her heart.
Birkhoff being tortured by Amanda in "Fair Trade"; not only does she brutally smash one of his hands, she then very nearly lobotomizes him, by inserting a blade into his brain through his nose. Easily one of the most disturbing scenes ever shown on the CW.
Michael is trapped under a burning car. To free him, Nikita cuts off his right hand.
The season 3 episode, "Intersection", reveals to us exactly what happens when a kill chip is activated when Amanda activates Sonya's, though it's thankfully a nightmare Birkhoff is having. The results are fairly disturbing.