fanatical

This word carries a heavy weight of obsession and rigidity. It describes a state where a person's devotion to a cause, hobby, or belief system has crossed the line from passion into an irrational or extreme territory. It often suggests a lack of balance or a refusal to consider opposing viewpoints. While it can be used playfully to describe a sports enthusiast, in political or religious contexts, it takes on a darker, more alarming tone. It implies a dangerous level of commitment that might lead to volatility or intolerance, distinguishing it from "enthusiastic," which remains positive and open.