People and situations are different. I am not so sure that it is cultural, unless there is some kind of religious element in the forgiveness or the vengefulness. It may just be a combination of personalities and circumstances that produce different results. In the American case of David Camm, I believe that his in-laws still think he killed their daughter and grandchildren and are fighting to keep him from getting life insurance benefits.
http://www.whas11.com/news/EXCLUSIVE-Frank-Janice-Renn-talk-about-David-Camm-verdict-230561041.html
The Kerchers were bamboozled by the prosecution with the help of Mr. Maresca early on. If they honestly believe Amanda had something to do with the murder, it is natural that they are angry that she is currently free.
They have a large financial stake in the outcome, which is probably a subconscious motivator to keep believing in Amanda and Raffaele's guilt. I agree pretty much entirely with Steve Moore's take on the Kerchers. They deserve compassion and understanding, but don't get a free pass to do whatever they want to whomever. *
http://www.gmancasefile.com/1/post/2014/01/-investigations-and-the-families-of-victims.html
The Sollecitos have lost a great deal, and Raffaele has been the only Sollecito who has mostly said that it's not Amanda's fault all the way along. I expect papa was frustrated that Raffaele didn't throw Amanda under the bus. Vanessa ended up losing her career, which is one more thing about this case that stinks to high heaven. It's natural that they are resentful, and still very afraid for Raffaele. I think that if he ends up in jail again and she is free, their resentment will be even greater.
http://groundreport.com/stolen-dreams-an-interview-with-vanessa-sollecito-2/
When people are hurt and afraid and angry, they lash out, and they don't always feel calm enough to sit down and make sure that they are lashing out in the right direction. The people who should carry the blame for this are Rudy Guede and the people whose professional responsibility it was to get the investigation and prosecution right and who failed to do so. Not Amanda. Not the Kerchers.
* The Kerchers have not done anything wrong that I know of. I am not saying that they are primarily motivated by money. However, I believe that it is naive to think that the money is a complete non-factor.