The only real piece of information we have currently about Fred’s death is Archie somberly narrating, “It hurts me that I never got to say goodbye” in the Season 4 trailer after receiving a phone call with the shocking news.
The trailer doesn’t offer much of an indication of what the culprit was beyond that. It’s very possible that, with Fred being out of town, he could have suffered a similar fate during his time away from Riverdale. That would make the most sense, after all.
Source : https://www.distractify.com/p/how-does-fred-andrews-die-riverdale