The Frog and the Scorpion
Sometimes it is difficult to see a person as a bully. The problem is that a bully does not act like a bully all the time. Bullies can be charming, smiling, and still have a stinger they keep hidden from the world. Bullies can smile and act like their target's best friend when it suits their plan.
All this can be confusing to the target of a bully and to co-workers. When the bully starts acting nice towards their target as if all had been forgotten and is in the past, the target may start to let their guard down. Perhaps the target begins to think the bully regrets their past behavior and has finally decided to change.
When the bullying behavior comes out again, it can be doubly devastating to the target. And the bully knows the affect that second betrayal will have on their target - they've done it all before.
Trust, once lost between two people, is very difficult to get back. This is why an organization has to do all it can to maintain the trust it's members have with each other -- and that's why a firm anti-bullying program is so important to the health of an organization.
Remember the story of the frog and the scorpion crossing the river? The scorpion promises not to sting the frog and kill it while they are crossing the river because the scorpion cannot swim and would drown.
But then the scorpion stings the frog while crossing the river, and when the frog asks, "Why?" the scorpion says, "Because it is my nature to sting."
So, the targets of bullies cannot blame their bully for being a bully -- it's who they are, after all.
It is up to the organization to rein in the bully's natural behavior for the benefit of all.