Birkin only leaked it because Umbrella's soldiers shot him and stole the samples. On top of that, they released unstable BOW's into the city to cover up the evidence, who then mutated out their controlled and tried to go on murder sprees to create more evidence against them. That, coupled with improper disposal of bioweapons qualifies as incompetence. Besides, if you sell people the virus to start the zombie apocalypse, what exactly would you plan to do with the money? The currency in the apocalypse is bottle caps so it's all worthless. There's no off button to stop the zombies once they've done their jobs as bioweapons. They keep spreading.The zombies are too hard to contain, the BOW's had a bad habit of mutating out of control after getting shot at too many times, and there was really no way that Umbrella could have come out of that business strategy with their finances intact. I'm sorry but zombie creatures just don't seem like a viable business move.
1) Birkin leaked the virus in the sewers long before he was ever actually attacked. He knew he would be, so he leaked the virus to cause an outbreak that would slow down Umbrella's agents. He then causes a second, larger leak after becoming a monster.
2) Umbrella didn't release a single "unstable" B.O.W. into the city. It released the Tyrant T-103, which was their best product. They also deployed Nemesis-T Type in order for it to demonstrate its effectiveness... and it did very well. They mutate out of their control after much damage they normally wouldn't receive. Vats of molten steel, chemical incinerators and magnetic rail cannons aren't going to be lying around.
They also deployed Hunters, but these were already one of their most effective bioweapons. There was no hope of any evidence being produced by the bioweapons in the city because Umbrella's political influence already cut-off all media within the city and blockaded it. Anyone who escaped was taken into custody by the U.S. Army, and either executed or recruited into a top-secret unit whose ulterior purpose was to prevent knowledge on bioweapons getting out. Then the missile wiped out whatever evidence was in the city itself.
3) Umbrella disposes of all their bioweapons properly (usually via incineration, whether chemical, molten or flames). The only time it didn't was when certain employees intentionally overloaded their processing capacity to cause an outbreak.
4) There is no "zombie apocalypse" in the games. Umbrella also hired mercenaries specifically for the purpose of clearing up any outbreaks caused by their bioweapons. These were also highly effective.
5) Zombies weren't sold as a bioweapon. They are also extremely easy to deal with if you have a decently sized military unit and
could be controlled (in a way) by the Tyrant, although I can't mention anymore about that right now.
6) No B.O.W. went out of control from mere shooting. It took a vat of molten steel for the Tyrant T-103. For Nemesis-T it took chemical incineration... and even then it retained its objective and could only be stopped after being blasted several times with a magnetic rail cannon... only to keep attempting to accomplish its objective before either being destroyed by a revolver or a missile.
