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It's a radiation hazard sortie, maybe you killed them accidentally, or killed an excavator.
In any case, it's best not to engage, right click -> report, don't say something that might get you reported and banned too.
What u/-n-k- said. Best to simply right click on their username and report. Or, you can make a ticket with Warframe Support and include screenshots, videos or any other helpful information. They handle these types of reports all the time.