The Baal Shem Tov (הוי זהיר במצוה קלה - near the beginning of Tzava'ath HaRivash for just one of many sources) says that we should burn brilliantly with our mitzwah performance. The Noam Elimelech (parashath Naso) explains that like a Nazir, when we really want to serve HaShem it should be to crown (נזר - crown) Him.
There's a level of mitzwah observance where we are doing everything right, and for good reasons, but where we are still aware of ourselves, still aware of the benefits we receive, still dilligent and watchful for our own wellbeing. It's almost impossible to not be aware of yourself, to not think of yourself at least in the smallest most basic and animal part of our minds.
We can be in the middle of our prayers, with a poisonous snake wrapped around our feet, and continue praying, telling ourselves that his is how I'm supposed to act, this is what the Hochamim tell me to do, I'm keeping the word of the Hochamim, I'm putting my faith in what is real. We can even know completely and totally that HaShem is the master of the world, and that we trust in Him completely. Yet through all this, there's still an I, still a self in there, still the focus is really, in the end, when you really get down to it, on us. The whole dilemma centers around our own existence, even when we define our existence as the ability to do more mitzwoth and serve HaShem for longer. Even if we are acting tirelessly with the knowledge somewhere in the back of our minds of knowing it will eventually be over and we will get a chance to rest, that is a focus and awareness of self.
First let me say that there is nothing wrong with this. May we all be blessed to reach such a level. Still, this is the difference between a Tzaddik and a Beinoni.
There is another level, and I speak from the limits of my best and brightest understanding, that is beyond this. Where there is no self, not from negation of self through meditation, or purposeful eradication of the self, but simply because the self was lost as a byproduct of our pursuit of HaShem.
At some point, the mitzwah is all that there is, the curled serpent doesn't exist, isn't part of the equation at all, and even the momentary temptation to be involved in something wrong is utterly inconceivable.
Let's talk a minute about this disgust a true tzaddik has for something evil.
This concept of mi'us, of disgust at something evil or wrong, cannot be built out of being disgusted with mundane or physical things. (Though we can and are meant to use this disgust as a learning tool.) Lehavdil, quite the opposite, no matter how physical or dirty and repulsive something might at first seem, we have to come to love it for the potential it bears to be used in the service of HaShem, in the performance of a mitzwah, the closeness it brings us to HaShem, the potential it has to glorify HaShem's name. The more we are focused on serving HaShem the more we love and appreciate everything in existence. The only thing that should repulse us is the desire to turn anything at all away from that purpose, the purpose of glorifying HaShem through His desires.
This is literally an impossible avodah. But, to really connect (without anything holding us back, ever) to HaShem who is, by definition, beyond us in every way, we need to cross the boundary of impossible.