This is probably the hardest thing for an author to recreate on the page. The majority of you will be decent people (more or less). But the same cannot be said of all of your characters.
The toughest things to write about are the most immoral acts: murder, adultery, manipulation, etc. What does your character think and feel when he does these things?
Writers draw on their personal experiences to answer questions like this one, but chances are you have never committed a homicide.
You can reproduce the way it happens through research and what you have learnt in news, film, TV and books. But this still does not help you get inside the culprit’s head. Do not expect a psychology textbook to be much help here. Each mind is unique.
Theft and manipulation are relatively simple, once you have the right approach. Did you ever lie, cheat at a game or steal a stick of gum? Take your thoughts and emotions in that moment and put them under a magnifying glass. Were you uncomfortable, guilty or excited? Amplify that to disgust, self-hatred and a sense of ecstasy.
We still end up left with the stickiest problem of them all: killing.
One approach you could take is to transplant the above feelings onto this act. If your character was disgusted by the theft they committed, murder might leave them absolutely revolted by themself and mankind. Or not.
There is another method. Once your character has gone past this point of no return, their moral compass might fall away. They have no reaction to what they have done, but simply go on to do it again. This can lead to a poignant moment in the narrative where they rediscover their humanity, confronting it through the clear lense of a third-party mentality.
Finally, you could look to reality for inspiration. Pick a notorious true criminal and identify from reports and newspapers what their reaction to their crime was. Base your character’s response on this, but do not copy it. Remember that you are telling the story of a fictional being which your imagination created, not the biography of a real murderer.