Alright. I'm gonna take a stab at this based on my own understanding and readings, and the assumption that you asked in good faith. We're going to break this up into a few things to answer your question.
The short answer is that murder/violence and cancel culture are not in the same box. It is a comparison of apples, oranges, and bananas.
The Merriam-Webster definition of cancel culture is: "The practice or tendency of engaging in mass canceling as a way of expressing disapproval and exerting social pressure."
Murder
The reason there isn't cancel culture (from my perspective) around this is that it is a direct violation of the social contract. You took a life, and it is up to the criminal courts to hold you accountable for the action. In essence, you're arrested for this and tried; you're already canceled, but by the state that's trying you.
Violence
Cancel culture does exist around this, but it depends on what is known. I believe that you only see it when there is a perception that the justice system has failed and the conscience of the masses thinks it needs to exact some sort of justice. That's where you see people being ostracized for domestic violence, accusations of sexual assault, etc. The criminal justice system hasn't acted on it for whatever reason, so others feel that it is their responsibility to do so.
The Moral Contract
Cancel culture is a social weapon that is used against people who are believed to have violated the moral social contract, and they are ostracized for those reasons. For example, Colin Kaepernick was canceled because he was kneeling for the Super Bowl (even though he didn't break any law, as the action is protected by the First Amendment), but because of the social moral conscience in 2016, his action was seen as an affront and thus had to be punished.
Hopefully this helped answer your question.