When a person sins against another, then he/she must ask forgiveness in order to get it. Also, when the offender asks for forgiveness from the offended...then the burden of the sin is transfered to the offended. If the offended gives forgiveness then the sin is gone. If the person does not grant forgiveness then God will not forgive the offended of his/her sins.

However, asking for forgiveness doesn't exempt someone from punishment. A debt must be paid...a recompense made.

In addition to asking for forgiveness, repentance is required. Repentance can be defined as "genuinely feeling sorrow for." In other words, you really have to mean it when you ask for forgiveness.

Example:

If someone murders one of my family members, I have the right to kill that person. If that person asks for forgiveness (and truly means it) then I am bound by the Law to forgive him. However, I am also bound by the Law to kill him for the sake of justice.

People (or angels) that knew God personally, not only sinned against Him, but they were unrepentant. This is why, most of what Plad posted earlier, God destroyed so many people.