Escalation rules generally allow anyone, including non-antags, to respond with equal force to the force being used against them. This means that, generally, anyone can kill anyone else in self defense.
As an antag, killing crew who are trying to kill you is very unlikely to be considered murderbone, regardless of how many crew are killed, including unintended deaths that are the direct result of an action taken in self defense. An example of “unintended deaths that are the direct result of an action taken in self defense” is people dying due to spacing caused by the antag to kill people attacking the antag.
This clarification is explicitly limited to killings in self defense. Killing an unarmed crewmember who has taken no threatening actions is not self defense and is not covered in any way by this answer.