At least if one is forced to do right, there is something good coming out of it. If you choose to be a monster, there is no right or good in it whatsoever. Only wrong. In fact I think it is worse than being forced to do wrong (which is obviously also wrong) against your will. If you choose to be a monster, there is something not only wrong in what you are doing, but something wrong in your conscience at some level to make you actually want to be a monster, and if it gets to a bad enough point it can become irreversible. This is why there are harsher sentencing guidelines for people such as serial killers who commit crimes of their own free will rather than under the orders of others.
From best to worst, I would rank the four possibilities set up by this choice as such:
1. Choose to do right, obviously
2. Be forced to do right
3. Be forced to do wrong
4. Choose to do wrong