Ok, so native Americans taking back America by armed force would be ok? Or areas of the world suddenly taking up arms and taking back areas around their border is suddenly ok (that's about the beginnings of world war 1 and 2).
The Falklands have been British for many generations (they've been British since 1833). The occupants are all British, and identify as being British. In votes, they chose to remain British, and actively oppose any attempt to make it otherwise. The UK was involved in negotiations to transfer the islands at Governmental level, but the now many-generations-established populace applied pressure to not allow this to happen.
The history is far more nebulous than "being owned by Argentina". If you apply proximity rules, then by your logic, Ireland should definitely be owned by England.
So, what you term "Evil" is actually supporting the wishes of a population that is well established (many generations over almost 200 years) who have lobbied hard to have their voices heard. These established occupants made it impossible to negotiate a transfer, so the only way to do it would be to evict them and force them off (which means you're pro forced resettlement of a population, which is really not a good thing).
The legal measures failed because the population of the affected area spoke their wishes.
So, not abiding by the established population's wishes, Argentina fell back to force, and sent in military to force the populace at gunpoint to claim what once was an unpopulated island (prior to England and France populating it back in the 1600's).
Have a good read of the history, and it turns out that Argentina has only very tenuous claim to the Islands.
Now, England has been allowing Ireland, Scotland and Wales to secede if they wish (i.e. the recent Scottish independence referendum). This is seen as a progressive measure.
If Argentina were to establish a claim on the Falklands, do you think they'd allow it to secede, despite the entirity of the population voting to leave? Would they hell. If they did, Falklands would secede at the first vote (they've made that plain), and would likely choose to join either England again as they historically have chosen to, or perhaps the EU.
So, no. What MT did in 1982 was exactly what she was supposed to do. Protiect British Citizens from invasion by foreigh force. Exactly what the island owners (the Falklanders, not Argentina) wished.