"Would have done" is the conditional perfect tense of "to do". It is usually used to express something in the past that did not occur conditioned on some other event. In this case, Jack is telling Rose that she is not going to commit suicide. Rose claims he can't know that. Jack replies "you would have done it already". Jack is saying that he knows she won't commit suicide cause if she really wanted to, she "would have already done it". In other words if she really wanted to kill herself why did she not already do it, because she actually doesn't want to..

The phrase "would have done it," is the same as "I would have done it." The first phrase is more casual and less formal. The first phrase is used more in verbal speech and the second is used in more writing. It means that I would have done something if I had known what I know now

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