Is there sin on those who say "Stop it" during a fight that led to someone being pursued, assuming their intentions were malicious or not malicious, and is it obligatory to report this to the police if one of the parties or another unrelated person dies?

Your liberation from whispers (waswas (definition)) is achieved by ignoring them and not paying attention to them, for they pose great harm. Occupy yourself with seeking forgiveness (istighfar), remembrance (dhikr (definition)), and reciting the Quran, for time is life, and wasting it on what is not beneficial is a loss. Allah and the Prophet have indeed warned about the importance of time.