In the wake of the Clone Wars, anyone who stumbled across a battle droid was understandably paranoid. After all, you never knew exactly when those droids might decide to exact a belated measure of revenge. Battle droids, however, were never as scary as assassin droids like the IG-88.
Immediately upon its initial activation (we're once again in "Legends" territory here), the IG-88 killed its human creators. Then he duplicated his programming into three identical droids, which set out on a quest to rid the universe of their organic overlords.
On the side, they took up bounty hunting. Darth Vader hired IG-88 to capture Han Solo's Millennium Falcon, which had proven more elusive than Vader expected. Their efforts to nab the Falcon failed, but IG-88 attempted to follow his real cause by uploading his programming into the Death Star, which he hoped to use as a beacon to call all droids in the universe to arms in a massive revolt.
Fortunately for humans and their carbon-based brethren, the Rebels blew up the Death Star just in time, saving creatures everywhere from the evils of the Empire ... and IG-88, too.