Christianity teaches that whenever evil is done, God had ample warning. He could have prevented it, but He didn’t. He could have stopped it midway, but He didn’t. He could have rescued the victims of the evil, but - at least in many cases - He didn’t. In short, God is an accessory before, during, and after the fact to countless evil deeds, great and small.
There's a difference between preventing an event and stopping an event in progress. In predestinarian traditions (e.g. Thomism, Augustinianism, Jansenism, Calvinism), there's a sense in which God cannot stop evil midway. In predestinarian traditions, everything happens according to plan. Once God implements a particular plan for the world, the series of events is unstoppable.
In that sense, although God can't stop a chain of events in midstream, God can prevent the outcome by implementing a different master plan. But once that plan is in place, everything happens like falling dominoes. (The same holds true for Molinism.)
This doesn't rule out petitionary prayer, for that, too, figures in the master plan.