I spilled the beans to my beta reader that the underlining romance plot was going to end in tragedy, more importantly death. This event happens in the first book, and acts as a catalyst for events to come in the following installments.
When an incident, be it death or something less permanent, happens to a character to advance the plot, then you are doing your job as a writer.
It bothers me a bit that your beta is giving you this advice on what you said rather than what you wrote. Finish your story and then let your beta (and hopefully more than one beta) read it and give opinions then. No one can know if a story works without reading it first. It is the way you write it that will tell in the end.