I partly agree about the part with the sudden shock. It is an option, but not the only one. Having him piece together what's going on until it builds up to his betrayal could also be another way to do it. Maybe make him have some suspicions, and then when he discovers his siblings have been killed it's the final straw for him. Don't make him immediately cut all ties with the protagonists once he turns to the other side. Maybe try to have him convince his old friends to join him by explaining his ideas on the matter. It would cause interesting chemistry between the protagonist and antagonist as both struggle to bring the other to their side. You could decide to eventually have their ties be cut after a couple of disagreements.
I partly agree about the part with the sudden shock. It is an option, but not the only one. Having him piece together what's going on until it builds up to his betrayal could also be another way to do it. Maybe make him have some suspicions, and then when he discovers his siblings have been killed it's the final straw for him. Don't make him immediately cut all ties with the protagonists once he turns to the other side. Maybe try to have him convince his old friends to join him by explaining his ideas on the matter. It would cause interesting chemistry between the protagonist and antagonist as both struggle to bring the other to their side. You could decide to eventually have their ties be cut after a couple of disagreements.
There are a lot of ways to pull the heel-turn off and it's ultimately up to what the writer feels would get the best response out of their audience. I like going for shock value myself, but subtle foreshadowing and build-up is something important for making the betrayal seem believable. You know, maybe the kind of stuff that a reader would look back on and think "Ah, why didn't I notice that before?"
