Steinbeck portrayed the bank as a great machine in which each person working for it played a small role that they didn't necessarily like to play. Agents were forced to evict people from their land because they were not able to pay their mortgage, yet that was not something that they really wanted to do. It was something they had to do because of the corporation they were a part of.
No individual person in the corporation is evil (generally). However the corporation is bigger than each individual member.
So calling a corporation evil does not mean branding each individual person. Rather it means that you recognize that the corporation as a greater force causes evil things.
In that respect I don't see it as unrealistic to say then that the corporation can be evil.
In addition, I see no reason why painting government or corporations as evil is a bad thing because when it comes down to it, what they are doing to third world countries is evil.
We execute murderers for killing people. Yet corporations are killing people overseas by poisoning and maiming them in dangerous factories.
I see no reason why calling a corporation evil is not accurate or not meaningful.