Jesus said, "You've heard it said, 'You shall not murder...but I tell you that anyone who is angry with a brother or sister will be subject to judgement'." Matthew 5:21-22
 
Not only is Will Smith right in that anger towards others can consume us (and destroy us, I might add), Jesus says that staying angry with others makes us subject to judgement. In other words, it's wrong. It's wrong to hold onto anger. It's wrong to hold a grudge. It's wrong to not offer forgiveness. 
Now I didn't say it was easy to let go of an offense, but it is wrong if we dont. The right choice is not always the easiest choice. But we can choose whether or not we are going to stay angry or hold a grudge. That is not just a mere emotion. It's a choice we make.