(Covers information from Anderson's universe)
Dr. James Marcus is an Umbrella scientist responsible for the creation of the T-virus, which could revive dead cells. But he disagrees to mass-produce it. Dr. Marcus also created the antivirus of the t-virus in case the t-virus escapes. Wesker later killed Marcus under Dr. Isaacs orders.
Wesker later killed Marcus under Dr. Isaacs orders, due to disagreements on the subject of mass-producing the virus.