Proverbs 28:17 is saying that as long as your are known as one who has killed it puts you in the eyes of others as some one who can be killed but it doesn't mean it would be the right thing to do. If it did then Moses never would have been God's chosen because he would have been killed, neither king David and others. They all had blood on there hands. The difference is they repented before God and His GRACE covered them because of their FAITH. They REPENTED & TURNED from their ways unto the ways of God never to repeat the same actions. This is REDEMPTION. So now anyone even the person who excuteS the leathal injection is quilty before God and blood is surley on their hands. If they practice this they have no covering and neither does their family if the don't know Christ. Once they REPENT and TURN from their ways unto the ways of God there is grace to cover them, but they cannot continue to murder as well. The bible also said not to render evil for evil. Remember God forgave us for our sins and our sins against Him are as great or greater than the sins we sin against each other.

I find it starnge that the court system use the Bible for people to swear to give the truth the whole truth, nothing but the truth but the lawyers and judge dosent consult the Bible or God on these matters, don't use the Bible as a show peace for the truth if it's not going to be read for the truth or punishment for the conviced.
Provebs 28:17 A man who is laden with the guilt of human blood Will be a fugitve until death; let no one support him. Ref: ( NAS Bible ). I beleive if the bible is used for counsle on these matters the convited won't be set free as much as they are today!