You can use
PLAYER_DEAD when the user has died. For raid/party members, I'd rather track their health by
UNIT_HEALTH to detect if/when they die than rely on addon messages. It's hardly secure by any means. Any knowledgeable user can block them or send fake messages. If you insist on using addon chat messages, at least check using the sender name from
CHAT_MSG_ADDON instead of including it in the message. At the very least, it'll provide a barrier from one person sending a message that a competitor has died.
I speak from experience of having made an addon message sniffer tool specifically to monitor messages going in and out of the local machine as part of a development toolkit.