Sorry he was running a illegal spice into the system. George himself wanted solo to be a drug mule but the studio voted it down
Spice was more in the class of steroids or coke, more except able to the studio.
There are several different kinds of spice in the known galaxy; they are all grouped under the spice rubric not because of any chemical derivations, but rather because they "spice up" the user in some fashion, either temporarily boosting physical strength and stamina, making the user feel invincible, or heightening awareness or creating euphoria. These "spiced" up abilities do not last long and the user eventually needs another dose of the substance and quickly becomes addicted.
Remember he was "forced" into running spice to pay off some old gambling debts to Jabba the hut who held the title to his ship.
Han Solo and Greedo both work for Jabba the Hutt, a crime lord based on Tattooine. Before the events of the film, Jabba put a bounty on Han, who had been smuggling cargo for Jabba, but was forced to dump it to avoid capture by an Imperial search party. In the Mos Eisley cantina, Greedo corners Han, points his blaster at him, and forces him to sit down in a booth. Han tells Greedo that he has the money to pay Jabba back, but Greedo demands Han's money instead. Han says he doesn't have the money at the moment, quietly readying his own blaster under the table. Greedo tells him that Jabba has run out of patience with Han and that he, Greedo, has been waiting for a long time to take out Han. Han replies, "yes, I'll bet you have." Lucus swears Han did not shoot first
His only option was to side with the rebels because he had nowhere to go. Later taking up the rebel cause.
Yea I am a geek

I am not a nuclear physicist, but play one on the internet.