That great parent they used to be can totally disappear, and they become totally self-absorbed, egocentric beings, with no filter on what is said or done in front of their children, or whether age appropriate. No consideration whatsoever, treating them like buddies, removing themselves from parental responsibilities. Oftentimes will ignore the children and act as if they don't even exist, even if a doting parent before BD.
Learn from yesterday, live for today, hope for tomorrow. The important thing is not to stop questioning.
Albert Einstein