In the First Age, Sauron was the Lieutenant of Morgoth. That's more or less like being Satan's right-hand-man. You don't get or hold a job like that without doing some pretty foul things. Sauron had some nicknames back then, e.g. Gorthaur the Cruel, Lord of Werewolves. Among his notable actions at this time were his nearly-successful attempt to kill all the Men of the House of Beor (which gave us Beren, and eventually Elrond and for that matter all the Dunedain).
In the Second Age, Sauron decided the world needed a Dark Lord, and since his old boss was cast into the Void outside of time, it fell to him to take up the reins. Or reigns, if you prefer. So he took Mordor and turned it into a vast breeding-ground for orcs. Meanwhile, using a disguise, Sauron lied to and corrupted the ring-wrights of Eregion, and nearly captured and enslaved the leaders of the elves. They found out his plans before he was able to catch the elves though. So since deceit was out, he assaulted and completely eradicated the Noldo kingdom of Eregion. That one was called the "War of the Elves and Sauron". Sauron slew Celebrimbor and took the Nine and the Seven Rings of Power for his own purposes. Feelings were hurt; elves can hold grudges for a LONG time. And, ok, the whole business with the cursed Rings of Power for Men and Dwarves didn't make him any friends, either.
At the end of the Second Age, Sauron lied to and corrupted the Numenorians, leading to their fall. Well, not just their fall, so much as the sinking of their whole island and the eradication of their civilization.
In the Third Age, there was - still - this lovely old forest called Greenwood the Great, which once covered most of northwestern Middle-earth. Sauron made himself a nice little fortress in the southern part of the woods, and his corruption and decay quickly spread. Now that forest is called Mirkwood.
Throughout the Third Age, Sauron's chief lieutenant, the Witch-king, continually assaulted Eriador, eventually bringing about the downfall of the Kingdom of Arnor.
Towards the end of the Third Age, Sauron started moving openly again, and gathered huge armies with which he intended to assault and subjugate all of Middle-earth. That's just not very friendly.
And, ok, Sauron also captured Thorin's father Thrain and tortured him to death, too. That was the last dwarven-ring held by the dwarves. Just adding insult to injury there.
So... on balance, Sauron was not a nice guy.