Was there any evidence given that Farage turned up on the day with a banner larger than that which the pilot had originally indicated was the biggest he could safely tow?
If Farage then insisted that the stunt must still go ahead because the election would not wait then you can see how the pilot might harbour a grudge. Having said that if the pilot believe that then he should certainly never have agreed to take Farage up in the plane with him.
Also threatening to kill the crash investigator (clearly only doing their job whereas one can see why the pilot might have reason to partly hold Farage to blame) and then calling the Police about the death threats does suggest that the balance of his mind was affected.
Also is Farage sueing this guy for personal injury or psychological trauma as presumably he has no valid insurance cover given that he was clearly in breach of his operating licence in carrying a passenger?