It's fine for him to voice his opinion and dissent.
However, any U.S. service member takes an oath to uphold and defend the U.S. Constitution. The U.S. Constitution grants authority to legislate. If new legislation is passed, he has already promised to uphold it.
Kind of sad that he forgot what he was trained to defend.
And seriously, "Comrade Feinstein"? Just because you disagree with someone does not entitle you to insult or defame them. You lose credibility with a lot of people by doing that.
Here's a book you might want to read.
How to Win Friends and Influence People