Re: Why we use ' would have done' on the following example?
There is no suggestion that the speaker's parent ever had to deal with a crime. The speaker is simply saying that his/her parents would have dealt with crime the same way if they had had to deal with crime at all. That is because they are talking about an "old-fashioned" way of dealing with crime, perhaps a way which would have been used when his/her parents were alive or when they were young.
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.