Jesus is speaking here to His disciples in Gethsemane. The Pharisees, led by Judas, are about to break in and arrest Him. You know the rest of the story.
"If I had not done among them the works which none other man did, they had not had sin: but now have they both seen and hated both me and my Father. But this cometh to pass, that the word might be fulfilled that is written in their law, They hated me without a cause." (John 15:24-25)
Dead to rights
There's a lot to this. When Jesus says "they had not had sin", what He's saying is, without a greater revealed standard to aspire to, there's no logical basis for conviction of sin. And conviction is always a good thing. Condemnation not so much. Jesus says He wasn't sent by God "to condemn the world" and that if one doesn't believe, they are "condemned already" (John 3:17, 18 respectively). Notice how the only sin He's citing in that passage is unbelief. As Jesus has come, God now has reason to convict those who don't believe.
"Furthermore we have had fathers of our flesh which corrected us, and we gave them reverence: shall we not much rather be in subjection unto the Father of spirits, and live? For they verily for a few days chastened us after their own pleasure; but He for our profit, that we might be partakers of His holiness." (Hebrews 12:9-10)
Also known as the hamartia, the tragic flaw is that thing in a person that, no matter how anfractuously the story winds (like a snake) and twists and turns, will always bring them down in the end. It's the downfall of the protagonist. What's so tragic about it is there's no escaping it. When first we sinned (whenever that was, it's different for everyone), it was...not acquired, but activated in us. The phrase "as soon as you're born, you begin to die" is apt and true. Both physically and spiritually. Paul speaks in his second letter to the Christians in Corinth of "the sentence of death in ourselves" (2 Corinthians 1:9). The next verse speaks of God "Who delivered us from so great a death".
In the book of Numbers, God tells Moses to "Make thee a fiery serpent, and set it upon a pole" (21:8). This, to counteract the epidemic of snakebites in the wilderness. Two verses prior does is say that "the Lord sent fiery serpents among the people" because of their incessant griping and whining and complaining. My opinion is , while it says plainly that the Lord caused it to happen, it means something more akin to "allowing" it. The idea behind anyone refusing to believe and how they are "condemned already" is that there is this construct in the ether where sin is dealt with automatically because this is God's creation. Paul says "that we might be partakers of His holiness". This creation is holy. And if we've believed on and in Jesus as the atonement for our sin, our spirit is holy as well. As an aside, it's funny how a similar but different thing symbolizes the medical profession. While the caduceus--from Greek myth--is a staff around which are wrapped dual, winged snakes. The simple, unpretentious "fiery serpent on a pole" symbolizes both Christ on the cross (yes: "For He hath made Him to be sin..." 2 Corinthians 5:21) and also, true healing. Weird.
Dead to wrongs
"For I was alive without the law once: but when the commandment came, sin revived, and I died." (Romans 7:9)
That's the same idea expressed by Paul as was spoken by Jesus. He talks about the commandment coming but it's actually good news. The good news is that there is a better way to do that which needs to be done, namely live. And this is what it means to be "dead to sin" (Romans 6:2). Upon accepting Jesus, we gain spiritual and eternal life. All feelings notwithstanding. It really is this simple. Is it easy? Sometimes. But most times it's excruciatingly hard to dig through the layers of old thinking that are now our responsibility to overcome. When once you push through that clingy and clayey top layer of soil, so to speak, you begin to grow and flourish by leaps and bounds. And if you're already dead and buried, there's no other place to go but up.
"Who can understand his errors? Cleanse Thou me from secret faults." (Psalm 19:12)
There's a branch of Theology that doesn't get much press. It's called Hamartiology and it deals with sin. What it is, how it affects us, etc. David prays in the above verse for a deeper understanding of what he might be doing to offend God. See, God loves us. He sent Jesus to pay for all our sin, so look to Him. God continues to draw us closer and we continue to molt, so to speak, shedding the layers of old, dead thinking that aren't in keeping with the recreated spirit that we possess because of Jesus' sacrifice. Don't be afraid to look at your sin (and sins) objectively. This might sound odd, but it's not who you are anymore. The caduceus may have two snakes, denoting duality. But the thing about Jesus is that you and He are one. And He left your sin in the grave when He rose from the dead.
"I in them, and Thou in me, that they may be made perfect in one; and that the world may know that Thou hast sent me, and hast loved them, as Thou hast loved me." (John 17:23)