In our law courts we have something called, "being an accessory to a crime." This particular aspect of our law (as many of them do) has its roots in biblical law. The whole idea of being an accessory to a crime has to do with knowing or seeing that someone has committed a crime - and saying nothing about it when called to testify against them. For our courts the one who is with someone while they commit a crime - even if they are not involved - can be brought up on charges of being an accessory to the crime. Charges like this are dropped when the one who has seen the crime testifies against the one who committed it. Some call this ratting someone out or being a snitch. Reality is that they are speaking the truth about someone who has a complete disregard of the law, public safety, and the cohesiveness of the fabric of our society.
To be a partner with a thief here means to be one who divides or shares in his actions. Proverbs 1 speaks of ignoring someone when they make an offer like this. The thief says that they will all share in one purse as they steal and kill to take what is not theirs. The same word is used in that passage as it used here. We are warned that someone who will do this with a thief (who will be their watchman to warn them if someone is coming - who is the get-away driver - who turns the other way when they break into someone's home or business) is someone who "hates" their own life. Hate is the Hebrew word "sane" and it means to be hostile to, loathe, or dislike intensely. When we ignore the warnings of God - His commandments - and indeed even those of our society - we do damage to our souls.
To be an accessory to a crime means we need to hurt our souls. We begin with our minds. We need to convince ourselves that God's laws and those of our society do not apply to us. We need to ignore the Word of God and its calls to us - as we ignore the Holy Spirit and our conscience. We then move to our emotions. Due to the Work of the Spirit in our consciences, we will begin to have adverse emotions. We will feel guilty (because we are guilty). We will also have to put to death things like mercy, compassion, and kindness in order to act in those ways toward whoever is being robbed. Finally, our will is affected. We set our course direction with our will and our choices. We intentionally chose against God's way - and in agreement with our flesh and the devil (who by the way loves it when we sin). Now we see the damage to our soul - or the way that we disdain the work of God within it.
The Bible speaks of hearing an admonition or oath - but saying nothing. This speaks of the Law in Leviticus 5:1 which says, "Now if a person sins after he hears a public adjuration to testify when he is a witness, whether he has seen or otherwise known, if he does not tell it, then he will bear his guilt." There it is plain and simple - the law against withholding information and in so doing helping a thief, murderer, or any kind of criminal who is acting outside of the law. Wisdom tells us that we should speak up on these matters - not to be a snitch or someone who is telling on everyone about everything possible - but rather as someone who desires for a society to hold together on the basis of law. This passage is speaking of clear cut crimes being committed. It also speaks of us being made aware of those crimes - being called to testify - and then choosing to withhold our testimony so as to be in league with the crime and the one who committed it. That is never wise. God's desire - and indeed the healthiest thing we can do for our souls - is to be a friend and ally of the truth. That is the position of wisdom - and of loving our souls, not hating them.