My doctrine is this, that if we see cruelty or wrong that we have the power to stop, and do nothing, we make ourselves sharers in the guilt. ~ Anna Sewell (1820-1878)
In the mid 1800s horses were not well treated, and Anna Sewell’s famous book, Black Beauty, lead to a change in public attitudes and, eventually, better treatment of horses. It seems sad that frequently, in workplaces around the country, and apparently around the world, people continue to be bullied or mobbed. If bullying 
and mobbing behaviours are naturally occurring human behaviours (and I believe they are) then it comes as no surprise that they do occur. What is an issue, however, is that colleagues watching the activity frequently stand by and do nothing. They look, smile sadly - knowingly - and do nothing.
It is not as though people don’t know what the behaviours are. They do. It could be than management could or should step in and sort it out, but they don’t/can’t/won’t. HR could or should step in and sort it out, but they don’t/can’t/won’t. Unions might, but rarely do, because the delegates are colleagues in the first instance, and they don’t/can’t/won’t. The people nearest, the ones who are seeing the target and the attacker(s), have three choices. They can step up to the mark and make a difference. They can join in with the attack. But what seems to be a very popular choice is for people to turn a blind eye to the plight of their colleagues.
This is not an innocent act, this is the face of guilt. The end result is the target leaves, the attacker(s) are left growling and ruffling their feathers, and the colleagues can barely look at each other knowing full well that if it happens again no-one will defend them either. They have become collateral victims, and the overarching atmosphere of guilt coupled with the tacit approval of inappropriate behaviours is demoralizing for all concerned.
The next step is the staff will create narratives to re-write history and provide justifications for the behaviours - note this is the avoidance behavior, not the bullying/mobbing behaviors - this is strategically ignored, or can even be justified - ‘we had to do this because X was a bad person’…
I overheard a conversation the other day about how …needed to be ‘performance managed’ out… in other words, someone had been targeted for removal from the pay roll. The other person nodded sagely, yes, that would be wise. Both parties had that special blank look protecting them from identifying the person as a person. Somewhere there was an unwitting victim about to suffer, somewhere there is team (unless some bold individual or two has stepped forward) also about to feel the guilt of inaction and subsequent demoralization.
When bad men combine, the good must associate; else they will fall one by one, an unpitied sacrifice in a contemptible struggle. Edmund Burke in Thoughts on the Cause of Present Discontents (1770)
, or my childhood favourite, 
. 
