You may do well to read the discussion under "Why did mormons abandon polygamy?" 
 
As well as "Are you sure Joseph Smith and Brigham Young are true prophets?" 
 
The Lord calls Imperfect men to lead his church and to act in his stead. They are still entitiled to their own opinions and views on certain things. They however, are not allowed to speak these Opinions and views, and declare them as official stances of the Lord's church. If they did, then the Lord would remove them from their place. I believe that the discussion on the Polygamy question will be sufficently answered through that discussion found under the question on this site where that is addressed. 
 
As for the Blood Atonement portion, that has never been declared as an official doctrine of the church. Of course people, have their own opinion, (I'm sure you have your own views and opinions when it comes to different aspects of your religion, (or your atheistic beliefs) as well.)which obviously has differed through the years, but has never been pronounced as Doctrine. When the Prophet is speaking for the Lord, the Prophet is 100% correct, 100% of the time (Thus saith the Lord).  
 
The times that the Prophet is speaking for the Lord, is declared as the official doctrine of the Church. Official Doctrine of the Church can be found in the Scriptures (Old Testament, New Testament, Doctrine & Covenants, Pearl of Great Price, Book of Mormon). As well as the things spoken from the Pulpit during General Conferences of the Church. 
 
Out side of these "Official Source" There have been many things that have been attributed to be a part of the Church's Doctrine, by those that are not allowed to designate such as Doctrine. To know if something is actually Church Doctrine, one would be wise to consult the Scriptures, as well as Conference Addresses.

Simply put, Practices can change, but Doctrine is eternal. 
 
Practices can change as the Lord reveals things through His servants, the prophets. This shows how important it is to have ongoing revelation; someone who speaks on behalf of God. Many points of the Law of Moses were practices that were fulfilled with the death of Christ. 
 
However, every practice originates from a doctrinal parallel. The doctrine of the Gospel of Jesus Christ is unchanging, even though true followers of Christ in each dispensation practiced according to the revelation they received. 
 
For example, in Old Testament times, sacrifices were offered in the tabernacle to remind the Children of Israel of the coming of the Messiah, and of the Atonement. In original Church, the sacrament was instituted as a reminder of that great atoning sacrifice. The doctrine of the Atonement remained constant, but as the time period changed, the practices changed. 
 
I'm glad that God continues to reveal the order of certain practices to living prophets, and I'm glad we have their words and the scriptures to establish the doctrines of truth.

Let me first state that there is some incorrect information in your question. Joseph Smith never practiced or condoned Blood Atonement that was purely of Brigham Young's teachings. 
 
These two issues have different explanations so I will tackle them one at a time for simplicity.  
 
First is why do we now condemn polygamy? 
 
The simple answer is we do not condemn polygamy, but lets go more in depth to full answer this question.  
 
Polygamy is a biblical practice that us governed by the laws set up by God in the various times in history.  
 
God re-established laws for plural wives in the time of Moses (Exodus 21:10). We know this practice was allowed before Moses' time because Abraham, Isaac, and Israel (Jacob) all had wives and concubines and were righteous men of God.  
 
In the Doctrine and Covenants section 132 at the very end of the section, the laws regarding plural marriage are reestablished in this dispensation of time.  
 
Plural Marriage was practiced in the Church until 1890.  
 
The reason for this discontinuance (not renouncement) of plural marriage is because the US Government outlawed the practice of polygamy in 1862. This law was challenged as unconstitutional and was decided by the Supreme Court in Reynolds V. United States to be constitutional in 1878.  
 
So now the Saints could continue the practice of polygamy illegally or obey the law of the land (which we are commanded to in D&C 58:21). 
 
In 1890 President Wilford Woodruff declared the Church would stop the practice of polygamy in obedience with the Law of the Land. New Plural Marriages were still conducted in secret without the consent of the First Presidency until 1906 when this practice was stopped.  
 
 
At no time has the Church ever declared plural marriage evil only illegal.  
 
 
Blood Atonement was a teaching of Brigham Young and was never declared by Brigham or any church leadership as a revelation or a doctrine of the Church.  
 
The President of the Church is no less mortal or capable of making wrong decisions than any other man or woman. This is why in order for doctrine to be established it must be done with the unanimous decision of the First Presidency and the Quorum of the Twelve Apostles.  
 
Any other practices outside of declared doctrine is not official Church practice but the practice or belief of an individual or group of individuals in the LDS Church.  
 
I hope that this helps you understand these issue better!

LDS guy - your comment is insightful, but not correct. The Prophet (President Gordon B. Hinckley) did in fact say that he condemns polygamy. This is from the Larry King interview. The exact quote is: 
 
I condemn it, yes, as a practice, because I think it is not doctrinal. It is not legal. And this church takes the position that we will abide by the law. We believe in being subject to kings, presidents, rulers, magistrates in honoring, obeying and sustaining the law. 
 
 
So the part where you say the church doesn't condemn it, well if Gordon B. Hinckley condemns it, then the church does. He claims that is is not doctrinal. So lets not sugarcoat our answers.

Nate, you have taken President Hinckley\'s statement Larry King out of context. He was referring to the unauthorized practice of plural marriage as carried on by certain splinter groups claiming to stem from Joseph Smith and Mormonism. President Hinckley was not condeming the practice if it is authorized/commanded by God, as it was in the early days of the LDS Church and in the days of the Old Testament patriarchs.