Ian’s entire career can be comprised of him taking shots at people when they’re at a severe disadvantage or when they are soft, almost completely defenceless targets.
This is what he does. He smiles to your face when things are looking good for you, he might even benefit from your relationship to some degree but when you stumble and fall as all people tend to do, when people are kicking you when you’re already down, he is quick to cut your throat. He’ll make a tidy profit doing it too.
It’s the mark of a man with poor character and dishonourable intentions to do things like that, instead of helping a man up and setting him right. Ian made a very big mistake thinking Sam was a mark, now he has to live with the consequences of people knowing what he is.