I don't think that the footage of the deadly terrorist truck attack should be shared with the public anymore, but I also don't think that it should be completely deleted. It will be helpful during the investigation, but I don't want to give the terrorists the attention that they want by sharing it any more.
The police should never delete the footage. This is because the country and the rest of the world needs to really see what terrorists are up to. If they delete it, it would seem they are trying to hide something from the French people, journalists, and even the whole world.
If French authorities need evidence of the attack and how it went down the footage could be very important. It would also be helpful to understand what happened in the attack because they may need to explain what happened to the people in order to kee them informed and so that they can use the information from the videos to prevent another attack from happening.
Although it may be heartbreaking to watch, prosecutors and law enforcement personnel may need to watch that footage for a possible trial or even as a case study. There's a risk that the footage may be used to incite further violence but law enforcement agencies need to prepare for such situations and analyzing the footage may be helpful in that,