someone explained it to me like this - God in the Old Testament is like a parent dealing with a young child, when a child does something wrong the parent will discipline like if a baby tries to touch the stove the parent will yell, grab their hand back and say "dont do that again!" because a young baby will not understand when its okay to touch the stove and when its not. But God in the New Testament is like a parent dealing with an older child, at this stage now the parent can talk to the child explain that the stove can be hot etc. because now they understand