LOL! That's...a bit of a leap, ain't it? After all, one can believe oneself righteous...and still do terrible things.Why is is wrong to talk bad about the recently deceased? Because even Atheists have subconscious understanding that there is an Afterlife.
All of us will face Judgement.
According to Talmud, someone who calls for G-d's Judgement on others will be judged more harshly themselves. Thus, a 100% observant and righteous person can judge others.
Judge not..lest ye be judged..always sounded like a warning..to me.