Revenge proves dangerous for Shylock because it leads ultimately to him ultimately losing everything and not just that but losing it really to the person he most desires revenge against, Antonio. We know that Shylock has just reason to feel aggrieved against the likes of Antonio, who freely admits to abusing him in the manner that Shylock describes, and we see other Christians ridiculing and mocking him in the course of the play, so there is no doubting the basis for his desire for revenge. When Antonio is forced to seek financial assistance from him, Shylock offers what he thinks is a perfect contract for him to exact this revenge should Antonio fail to honour his side. Unfortunately, when Shylock goes to court the odds are stacked against him, not just because he is an outsider against a Christian establishment but also because the judge is bogus and has a vested interest in finding in favour of one side only. On top of all that the judgement handed down is ridiculous in its pedantic insistence on no blood and the weight of flesh being precisely as stated, not a jot more or less than required by the bond. Shylock is destroyed utterly as a result of his desire for revenge, or perhaps we should say because of his failure to see that, no matter how good a case he might have, his pariah status in Venetian society means that he can never win against a Christian.
Start your essay with a broad statement along these lines and then develop it by referring to the text, mainly the events that bring about the bond and then the trial and judgement. Good luck.