By default, if a death fact is in the database, it will be shown on a chart as “died” or “d.” depending on the style of the chart. Sometimes, it might be preferable to show “Killed in action”, or use a different verb.
Ensure that the person is selected, and then locate the death fact from the fact grid. Use the “Type” box to choose a different kind, such as “Killed in action”
Not all types here will change the text on the chat, but many will.