如果我發現有人偷拍人裙底, 我破壞了那人的手機或是相機 在法律上我用不用賠償給那人?

不會。但視乎法官的判決。
第一，被告的手機是用作犯罪用途；第二，為防止罪案繼續發生，受害人所做的行為算為合理。
這跟把準備用槍殺人的人的槍破壞一樣，沒有槍牌卻擁有槍己經是罪行，槍本身己不是合法的存在，而槍亦是危害大眾的事，被破壞又何罪之有？同樣地，那手機雖是私人財物，但卻是用作偷拍的犯罪行為之用，亦是影響及危害大眾的事，被破壞是為了防止被告繼續犯法及危害他人，相信就算被告向原告或受害人索償，法官也很難會讓被告成功。

法律上是不用賠償, 但是......
捉人偷拍是一件難事~ 如果無憑無據, 好難入人地罪~ 仲要賠償同埋被人反告你毀謗~
如果你破壞人家的東西而內裡還殘留一D證據, 都未必可以入佢罪, 因為你仲要證明佢蓄意偷拍先有機會告得佢入~ 所以人證好重要, 但係現今社會人證難求, 只有真正的好心人先會花依D時間同心機幫人做人證~
所以做好事前都必須三思呀!

In the actual court of law, you will be charged for destruction of private property, destroying evidence or obstruction of justice.

You have to understand that the destruction and the photo-taking is two separate incidents. You may say that if he does not do that, you won't do that. It is logical correct. But remember, by using this logic, you can say because A kill your friend, so you have to kill A for revenge. Law does not work like this.

However, in the reality, most people won't press charge because the victim won't press charge against the suspect.

One thing you have to realize that, the Department of Justice is the one who presses charges, not you because the crime is a matter of public interest.

是要的,因為你的確上破壞了那人的東西

是要的。因為事主/被告或會向你索賠，結果兩者皆應承擔代價。