(Psychological / emotional perspective)
It can be important in a dream about a knife to notice what type is being used.
For instance, a table knife would be interpreted very differently to a swiss army knife. Both are functional, but the former would only be appropriate under certain circumstances, whereas the latter might have a more universal application.
To dream that we are carrying a knife signifies anger, aggression and / or separation. We may be attempting to cut ties or feel that we need protection.