There are a variety of phrases that can be used as synonyms for "went off the deep end". Some common options include "lost it", "went berserk", "had a meltdown", "lost their cool", "went crazy", "lost their composure", and "flew off the handle". Each of these phrases suggests a sudden and intense emotional outburst or breakdown, likely in response to a triggering event or situation. Whether used to describe a personal experience or to add color to fictional writing, these synonyms can help convey a sense of dramatic intensity and emotional instability.