I think there is a case for 'good behaviour' when players who are very clean make a one-off, bad tackle.
I wouldn't put Farrell in that category at all. He has plenty of form for bad tackles, he just hasn't been adequately punished for them. Not only are they dangerous, but they're also entirely deliberate. He may not have intended to make contact with Atkinson as he did, but when you come in that high and that hard, you have no defence IMO.
It's been a ticking time bomb and hopefully it leads to a change in his behaviour.