At the beginning of the Resident Evil movie, we see somebody stealing from The Umbrella Corporation. Is there an in-universe answer as to who the T-Virus thief is planning on selling the T-Virus and the antidote to?
As I recall, the theft was done by several people:
Alice (main character) worked as security for the Umbrella Corporation, and was a contact for the thieves. She supplied schematics, floor plans, patrol times, "the works" as she would say.
Lisa is an activist, managed to infiltrate the company in order to steal the T-Virus so that they could expose the atrocities at the corporation and take it down.
Matt is Lisa's brother, and also an activist. He is the one that pretends to be a policeman and goes along with Alice and the team down into the facility. His mission is to find out what happened in the facility, see if he can retrieve a sample of the virus, and discover what happened to his sister.
So the theft was not done by another company or for financial gain, but by a small group of people in order to bring Umbrella's "research" into the public eye.
EDIT: Thanks to phantom42: There is a second theft: Spencer, who is the one that you mentioned in the question. He was apparently working together with Alice (who in turn was working with Lisa) to steal the virus as well. The formula was going to be sold "on the black market", so there is no way to know if there was another company or government organization behind that theft.
With the last installment of the film series being released earlier this year, it's time to clean up some misconceptions and bring the missing pieces to the puzzle.
To answer your question, the T-virus thief is Spence, a security operative for the Umbrella Corporation. He worked undercover together with Alice in an arranged marriage in order to protect the secret entrance to the hive at their mansion. Alice got involved with Lisa, a disguised employee at Umbrella and an environmental activist, who, together with her brother Matt, planned to bring the corporation down and expose it to the press.
Spying on Alice and Lisa, Spence discovered their secret plan to infiltrate the Hive in order to steal the T-virus and expose the unlawful research conducted in Umbrella's labs. However, he didn't share their heroic aspirations and believed that the mission was deemed to a failure, given the enormous power that the corporation held at the time. Instead, he entered the Hive ahead of Alice, stole the T-virus, and released it to the air, so as to facilitate his escape.
Besides bringing chaos to the labs as a means of escape and not getting caught, Spence also (allegedly) broke the vial to make Alice's dreams of bringing the corporation down come true (hence the quote, "Today all your dreams come true"). Nonetheless, his primary motivation was to sell the virus for a high price on the black market, so that he and Alice could run away and enjoy a prosperous life together. It is not clear however, as to who the virus was planned to be sold to specifically or how the profits were to be made exactly.
The Final Chapter (2016)
Now that the last movie has been released, we know that Alice was actually a clone created by Umbrella itself. It's possible that Spence was a clone as well, in which case one might argue that Spence was actually instructed to steal the T-virus and initiate the massive outbreak. This is backed up by the fact that Doctor Isaacs himself wished for the virus to be released in order to bring about an apocalypse and rebuild the world. It's possible that he ordered Spence to release the virus just before Alice, Lisa, and Matt got hold of it, and before the corporation could be exposed to the media.
I just watched the first (2002) and the last (2016) movies, but there are also plenty of good sources online that might shed some light on this subject. There're more details to this question obviously, but I skipped them for the same of brevity. For more info on why Spence broke the vial, check out this question.