What prevents Pip from safely getting Magwitch out of the country? Who is responsible for the impediment and why happens to this person?

Do you mean near the end of the novel? They start off the next day and are within a few feet of a steamer that they hope to board when another boat pulls alongside to stop them. In the confusion, Pip sees Compeyson leading the other boat, but the steamer is on top of them. The steamer crushes Pip's boat, Compeyson and Magwitch disappear under the water, and Pip, Startop and Herbert find themselves in a police boat of sorts.

Magwitch finally comes up from the water. He and Compeyson and wrestled for a while, but Magwitch let him go and now Compeyson is presumably drowned. Once again, Magwitch is shackled and arrested. Magwitch dies shortly after in prison.