If the Biblical law tells me, you shall not murder and U.S. laws are against murder, if Jesus in his Sermon on the Mount tells me I should not murder or be angry with my brother, then I should not murder. Arguing whether it is advice or a commandment is a waste of time.
What you say is right.
And, in general, even an atheist observes the last 6 Jewish commandments.
For example, an atheist refuses to be a killer to gain more money (or else) for himself, though he doesn't have a god to tell him not to kill.
In spite of this, in the material world, one may be allowed to kill (if not worse) if he does it while he serves his ruling system (as in wars).
I am afraid that 'Love your enemies' (which contradicts one's instincts of survival and world's rules) is far beyond any imposed rule.
Here is an experience I lived:
During my military service, many decades ago, I was an officer (of the lowest grade) being an engineer. One day, I was asked to join a training session (of small guns). I simply refused to participate, so there was the following conversation:
They: This is done for your benefit so you can defend yourself when necessary.
Kerim: Thank you, but I have no enemies in my life.
They: You may not have enemies. But there is always the possibility that certain enemies invade our country, sooner or later.
Kerim: Sorry, what do you mean by 'enemies'?
They: They are those who will impose their will on us by force.
Kerim: Do you mean like what you try doing with me now?
They smiled and left me alone. I was never asked again to join such session. During my service, I just taught electronics in a military academy.