You definitely should not harass someone because they have been exposed as abusive online, but you should not eagerly try to dismiss the victim as “too evil to have actually been abused” the way people are doing with Eron.
Abuse is not something that happens only to Good And Innocent Sweeties (even Stalin had abusive parents) and it is very harmful to assume that anyone you consider immoral in some way just made everything up.
There is a problem in that abusers can adapt to “always believe the victim” memes by accusing their victims of being abusive and directing online mobs against them.
So the safest response seems to be that, no matter who you chose to believe, you should not form online mobs against people.