Why is worship of the Nine Divine god Talos illegal in Tamriel, now? Has it been explained in depth yet by Bethesda, or is it something we are going to have to find out ourselves in the game? There must be a pretty important reason behind the sudden, unlawful worship of the former divine Talos.
First of all, read up here on the history of Tamriel: http://www.imperial-library.info/content/tamriel-timeline
I would also suggest spending some time in the Elder Scrolls Lore forum.
But I'll sum up for you as best I can. The Thalmor are evil elves who want to do more than just obliterate mankind. They want to make it so that we can't exist. They want to erase the possibility of our existence. They think that they are really gods who have been forced to embody flesh and blood bodies in this world. They want to return to the cosmos and be gods again. For some reason, getting rid of even the possibility of man makes that possible, or at least they think it will.
But what does Talos have to do with it? Well, Talos was a dragonborn MAN who ascended and became a god. He symbolizes man's ascendency somehow. In Tamriel, if people worship Talos, then he exists. If they don't worship him, he ceases to exist. In Tamriel, beliefs shape reality. So perhaps if Talos ceases to exist, man will not be able to ascend anymore, which will help clear the way for the elves.
It's kind of complicated, I know. But it all started way back in the beginning of the First Era, when Nords first began to settle in Skyrim. The elves, who had been there first, became concerned that man would become too powerful and take over. So they burned the Nordic capital to the ground in an event now known as the Night of Tears. Ysgramir and his two sons were the only survivors. But Ysgramir came back with his famous 500 Companions (whose descendants now populate the Companions Guild) and defeated the elves, and the two races have been bitter rivals ever since.
Perhaps the elves' jealousy of man stems from a theory they might have that man's dominance of the land will keep them from becoming gods...I'm not sure on that point, but whatever. You get the idea.
All of this background history becomes somewhat important to Skyrim. Well, it's important if you care about lore. But if you want to learn more about it, there will be lots of in game books and people to talk to, etc.
Bottom line: the Thalmor are evil high elves who are trying to erase mankind, and banning the worship of Talos helps them to achieve that goal. Your response: kill them on sight.
