This is not complicated at all if you know Christ as your Saviour and are born again and indwelt with the Holy Spirit to show you what His word means. Paul is saying that believers in Christ are justified by faith APART from the works of the law. It is actually a very simple concept. He is saying that the works of the law DO NOT justify anyone. Yes, God wants us to keep His commandments. Are we able to keep His commandments? No. Does God know this? yes. So He sent His Son to be the fulfillment of the Law and the prophets. Does this mean that once we accept Jesus Christ as our Saviour that we should do anything we want to do? Of course not. We should do what we know is right and the Holy Spirit who indwells all born again believers will show you what is right. Will we still sin (miss the mark and break the law)? Yes, but if we confess our sins then Christ is faithful to forgive us our sins and cleanse us from all unrighteousness. He died for all sins, past, present and future, but we still need to go to Him daily and repent of our daily sins and ask His forgiveness. Yes, the soul that sinneth, it shal die. We all have souls and we all sin. We all will die physically, but only those without Christ will die spiritually. This is the second death which is hell/the lake of fire.

You are also confusing law and grace. The law was given to Israel to show them their need for a Savior, because God knew that no man could keep His law. The law was given to Israel as a schoolmaster to point them to Christ. Jesus was born under the law and He came to fulfill the law and the prophets. He is the fulfillment of the Old Testament, the prophecies and the Law. Grace is unmerited (unearned) favor that God gives us for believing His word. Abraham was before the Law, but he believed God and God credited it to him for righteousness. Those who were under the Law and believed God's word and tried to follow it the best they could, they too were saved by God's grace through faith. Those who did not believe God's word but only followed the Law to be seen by other men, were not saved by God's grace because they did not have faith in God's word. They only followed the Law out of duty because it was what they believed they were suppose to do. It is the same today with all "religions" that think they can obtain God's grace and mercy by doing good works. God wants us to believe His word, not just do good works.

Again, let's look at the verses you quoted from Matthew in context.
Matthew 19
16And, behold, one came and said unto him, Good Master, what good thing shall I do, that I may have eternal life?

17And he said unto him, Why callest thou me good? there is none good but one, that is, God: but if thou wilt enter into life, keep the commandments.

18He saith unto him, Which? Jesus said, Thou shalt do no murder, Thou shalt not commit adultery, Thou shalt not steal, Thou shalt not bear false witness,

19Honour thy father and thy mother: and, Thou shalt love thy neighbour as thyself.

20The young man saith unto him, All these things have I kept from my youth up: what lack I yet?

21Jesus said unto him, If thou wilt be perfect, go and sell that thou hast, and give to the poor, and thou shalt have treasure in heaven: and come and follow me.

22But when the young man heard that saying, he went away sorrowful: for he had great possessions.

This was a rich man who came to Jesus and asked Him how he could inherit eternal life. Jesus asks the man why he calls Jesus good because only God is good. So Jesus tells the man if he wants to have eternal life then he needs to keep the commandments, and He names the common laws, the basic commandments that most people obey. The rich man tells Jesus that he has kept these laws from his youth, but asked what he still lacked.

21 Jesus said to him, “If you want to be perfect, go, sell what you have and give to the poor, and you will have treasure in heaven; and come, follow Me.” 22 But when the young man heard that saying, he went away sorrowful, for he had great possessions.

Jesus told the man to give up the things of this world and believe in Jesus, but the man did not want to follow Jesus because he wanted his money and the things of THIS world NOT eternity.

Jesus goes on to tell his disciples
29 And everyone who has left houses or brothers or sisters or father or mother or wife or children or lands, for My name’s sake, shall receive a hundredfold, and inherit eternal life.

You see, Jesus/God knows that when you follow Jesus, then the world will turn against you. Jesus said that the world hated Him and will hate those who follow Him.