It was a direct attack and violation of sovereignty of Ukraine. Do you remember what Russia promised in Budapest Memorandum on Security Assurances?
Once Ukraine had the world's third largest nuclear weapons stockpile, they give up this arsenal for the promise of territorial integrity and assurances against threats or use of force against them by Russia.
Crimea was a part of the Russian Federation until 1954, and the population is overwhelmingly ethnic Russian. In 1954, the region was forcibly merged with Ukraine. So there is nothing wrong in Crimea returning to Russia.
And regarding nuclear weapons, Russia is regarded as the successor nation to the USSR. Only they were allowed to keep the nukes, and the other nations such as Belarus, Ukraine, Kazakhstan.etc gave up their weapons.