You assume that, video games, movies, and other media have desensitized people to the point that something horrendous would be "normal". The description would also have to be pretty well done. The more in-depth you describe the situation/event, the more the reader can "feel" the scene. Do you set the right tone? the right mood? do you describe the environment and the actions? How do yo describe the actions? Do you talk about the MC's thoughts as they are going through the scene?
In my opinion, if you need to provide "extra hints", you didn't describe the situation well enough. Take something as stupid as a doll. No one is scared of a doll. But with the right tone, the right description, the right actions applied to the doll and all of a sudden we have stories like chuckie and saw.
To make the reader uncomfortable would mean that you also need to make the MC uncomfortable with their emotions. Most people when they read a good story put themselves as part of the scene, as part of the MC. Make people FEEL that animalistic/predatory urge, that raw emotion deep down. Bring it out of the reader so that they feel just like the MC as their morality and self values will automatically create the feeling you are looking for.
People know that violent acts are intrinsically wrong when it is not out of the necessity to survive. Anyone who feels otherwise have chemical imbalances. You don't need to tell people that killing someone else is wrong. Everyone knows it is no matter how "desensitized" you may think society is. That is why murder is a crime globally and not based on social/cultural influence.
So you don't need to TELL the reader this is wrong nor do you need to drop hints. You just need to describe the scene with enough detail, tone, mannerisms, and MC thoughts/feelings that they will feel uncomfortable.
Having sex with a dead body is weird but simply stating he had sex with a body he just killed won't make anyone uncomfortable. Describe it, in detail. Describe the sounds the body makes, any excretion of body fluid as he does his things. Use your imagination to bring the scene to life.