The usual definitions of terrorism (of which there are many of varying degrees) involve an intention to promote political or idealogical change through the use of 'terror tactics' (eg. explosives campaigns; public assassinations) to force either Popular or Government support on the matter. In the Resident Evil franchise, bioterrorists such as Il Veltro also go by similar definitions (albeit through the use of biological weaponry over explosves). True, however, there are people whose idealogies involve the complete destruction of current human civilization, to be succeeded by their own world.
The intentional spreading of an exceptionally-hazardous virus such as the t-Virus would, in the late-20th century United States, be comparable to knowingly infecting people with HIV. This act in itself is not generally viewed of as bioterrorism (some leeway may be given if it was the '90s and you slept with President Clinton) but as a First Degree Murder or Criminal Negligence. So, for someone to infect the staff of the Arklay Laboratory and later infect an Umbrella train, we're looking more closely to Homocide acts such as Mass Murder than more-defined acts of Terrorism.
I would then like to bring up another fact - the antagonist of Resident Evil Zero is not really James Marcus. As shown in a cutscene towards the end of the game, Dr. Marcus was murdered in 1988 and his body dumped into the waters of the Treatment Plant. It was his Queen Leech which merged into his being. As such, this isn't the sound-of-mind Dr. Marcus of the '80s performing these acts, but a t-Virus organism walking around with his body. Think of this in the case of Javier Hidalgo when he merged with the V-Complex - he wasn't the one trying to kill Manuela but the V-Complex taking over his mind.