DISCLAIMER: Since I'm referencing some basic legalities I need to inform anyone reading the following that I am NOT a lawyer, legal consultant, or paralegal. This is NOT legal advisement or advice and should not be understood as such. If you need official legal advice on Self-Defense seek out a licensed lawyer or your state attorney general's office for clarification on your state and local ordinances regarding the subject of Self-Defense.

In real life mercy is called restraint. Most martial artists are taught(are supposed to be taught) to use escalation of force from the very start to prevent unintentional deaths and most states have laws stating something like

"you have the right to protect yourself and stop the threat but once the agressor(s) are no longer a threat (knocked out, grievous injury preventing them from fighting etc.) the you must stop or else you will also be charged with battery".

this is a very very very simplified and non specific version of those laws and you should look up your own state and local ordinances on Self-Defense laws and practices.

Source: I taught Self-Defense before the pandemic(and hopefully I'll be lucky enough to continue to do so afterwards)