Hitler adopted the swastika as the symbol for his Nazi party because it originally represented peace. He saw this as a message to the German people that his party was to end all the turmoil and unrest in Germany at the time. There was of course at the time a lot of upheaval and poverty in Germany still lingering as a byproduct of the loss of the first world war. The instability came then from the various political and workers parties tried to seize power from the ineffectual government put in place in the wake of the first world war.
After his rise to power and the actions he took, the swastika has become a symbol synonymous with racism and hatred, and even to this day will conjure up thoughts of the holocaust or Nazis to anyone asked. Hitler originally chose the symbol apparently because it was the symbol used for his mothers headstone, which was a common gravestones symbol in Germany and Austria at the time, whom he idolized.
The swastika is now illegal to display in many western countries because of this association, and if you were to display the swastika anywhere people would immediately assume you had extreme right wing views. It is illegal to have a tattoo of a swastika for example if you want to join the police force or the military as well.
The association of the swastika as a symbol of fascism alone is of course an unfair representation of the symbol, which can be found in many locations globally dating back for hundreds of years. It is not known exactly whee the symbol itself comes from, although the word swastika was originally from Sanskrit. There are several theories about the original existence of the swastika, although there are examples of it going back as far as 5000 BC, and perhaps even further.
The swastika can still be seen in certain places today, (used in its original context of course) such as in common usage in Japan or in numerous Buddhist temples, as the image has no immediate negative meaning there as they weren’t affected by the Nazis being that far away from the scene of world war two. In fact other then western countries the swastika is still a common sight with a lot of different meanings.
The swastika is also an important symbol in Hinduism. A right facing swastika represents the development and evolution of the universe, and a left facing one represents the destruction and involution of the universe. There are also several other religions that use it as a prominent symbol, such as Jainism.
So basically Hitler used the swastika because it was a popular symbol of the time, and as a dedication to his mother. He saw it as a representation of the intentions of the Nazi party in uniting Germany again. It would be similar to a maniacal dictator today using the symbol from the red cross as a crest to give a similar example. The symbol in itself was only looked upon negatively after the second world war, and before this it was an acceptable pattern to use in a multitude of things.